diff --git a/src/vs/base/browser/ui/tree/abstractTree.ts b/src/vs/base/browser/ui/tree/abstractTree.ts
index 39a7a0a2cd1579290b4fe12f6998c7f72e5f6ece..3ec13bfdcf1ef89655ffde1dc8206a60c4af487f 100644
--- a/src/vs/base/browser/ui/tree/abstractTree.ts
+++ b/src/vs/base/browser/ui/tree/abstractTree.ts
@@ -288,15 +288,15 @@ export abstract class AbstractTree<T, TFilterData, TRef> implements IDisposable
 
 	// Tree navigation
 
-	getParentElement(location: TRef | null = null): T | null {
+	getParentElement(location: TRef): T {
 		return this.model.getParentElement(location);
 	}
 
-	getFirstElementChild(location: TRef | null = null): T | null {
+	getFirstElementChild(location: TRef): T | undefined {
 		return this.model.getFirstElementChild(location);
 	}
 
-	getLastElementAncestor(location: TRef | null = null): T | null {
+	getLastElementAncestor(location: TRef): T | undefined {
 		return this.model.getLastElementAncestor(location);
 	}
 
diff --git a/src/vs/base/browser/ui/tree/indexTree.ts b/src/vs/base/browser/ui/tree/indexTree.ts
index fa6a3ad5e9ebda5183e1ce99da47cb6a7bfb8196..d8e56d81167bae24518893b9a3f718adc1452938 100644
--- a/src/vs/base/browser/ui/tree/indexTree.ts
+++ b/src/vs/base/browser/ui/tree/indexTree.ts
@@ -23,6 +23,7 @@ export class IndexTree<T, TFilterData = void> extends AbstractTree<T, TFilterDat
 		container: HTMLElement,
 		delegate: IListVirtualDelegate<T>,
 		renderers: ITreeRenderer<any /* TODO@joao */, TFilterData, any>[],
+		private rootElement: T,
 		options: IIndexTreeOptions<T, TFilterData> = {}
 	) {
 		super(container, delegate, renderers, options);
@@ -33,6 +34,6 @@ export class IndexTree<T, TFilterData = void> extends AbstractTree<T, TFilterDat
 	}
 
 	protected createModel(view: ISpliceable<ITreeNode<T, TFilterData>>, options: IIndexTreeOptions<T, TFilterData>): ITreeModel<T, TFilterData, number[]> {
-		return new IndexTreeModel(view, options);
+		return new IndexTreeModel(view, this.rootElement, options);
 	}
 }
\ No newline at end of file
diff --git a/src/vs/base/browser/ui/tree/indexTreeModel.ts b/src/vs/base/browser/ui/tree/indexTreeModel.ts
index d57e98846cba4f07d725cd94a654a883a10fb47a..d23626cdc6251a7b075b00f122e8b1a1a0480ca1 100644
--- a/src/vs/base/browser/ui/tree/indexTreeModel.ts
+++ b/src/vs/base/browser/ui/tree/indexTreeModel.ts
@@ -43,20 +43,9 @@ export interface IIndexTreeModelOptions<T, TFilterData> {
 	filter?: ITreeFilter<T, TFilterData>;
 }
 
-export class IndexTreeModel<T, TFilterData = void> implements ITreeModel<T, TFilterData, number[]> {
-
-	private root: IMutableTreeNode<T, TFilterData> = {
-		parent: undefined,
-		element: undefined!,
-		children: [],
-		depth: 0,
-		collapsible: false,
-		collapsed: false,
-		renderNodeCount: 0,
-		visible: true,
-		filterData: undefined
-	};
+export class IndexTreeModel<T extends Exclude<any, undefined>, TFilterData = void> implements ITreeModel<T, TFilterData, number[]> {
 
+	private root: IMutableTreeNode<T, TFilterData>;
 	private eventBufferer = new EventBufferer();
 
 	private _onDidChangeCollapseState = new Emitter<ITreeNode<T, TFilterData>>();
@@ -68,9 +57,21 @@ export class IndexTreeModel<T, TFilterData = void> implements ITreeModel<T, TFil
 	private collapseByDefault: boolean;
 	private filter?: ITreeFilter<T, TFilterData>;
 
-	constructor(private list: ISpliceable<ITreeNode<T, TFilterData>>, options: IIndexTreeModelOptions<T, TFilterData> = {}) {
+	constructor(private list: ISpliceable<ITreeNode<T, TFilterData>>, rootElement: T, options: IIndexTreeModelOptions<T, TFilterData> = {}) {
 		this.collapseByDefault = typeof options.collapseByDefault === 'undefined' ? false : options.collapseByDefault;
 		this.filter = options.filter;
+
+		this.root = {
+			parent: undefined,
+			element: rootElement,
+			children: [],
+			depth: 0,
+			collapsible: false,
+			collapsed: false,
+			renderNodeCount: 0,
+			visible: true,
+			filterData: undefined
+		};
 	}
 
 	splice(
@@ -353,7 +354,7 @@ export class IndexTreeModel<T, TFilterData = void> implements ITreeModel<T, TFil
 	}
 
 	// cheap
-	private getTreeNode(location: number[] | null, node: IMutableTreeNode<T, TFilterData> = this.root): IMutableTreeNode<T, TFilterData> {
+	private getTreeNode(location: number[], node: IMutableTreeNode<T, TFilterData> = this.root): IMutableTreeNode<T, TFilterData> {
 		if (!location || location.length === 0) {
 			return node;
 		}
@@ -418,7 +419,7 @@ export class IndexTreeModel<T, TFilterData = void> implements ITreeModel<T, TFil
 		return location.reverse();
 	}
 
-	getParentNodeLocation(location: number[]): number[] | null {
+	getParentNodeLocation(location: number[]): number[] {
 		if (location.length <= 1) {
 			return [];
 		}
@@ -426,33 +427,33 @@ export class IndexTreeModel<T, TFilterData = void> implements ITreeModel<T, TFil
 		return tail2(location)[0];
 	}
 
-	getParentElement(location: number[]): T | null {
+	getParentElement(location: number[]): T {
 		const parentLocation = this.getParentNodeLocation(location);
 		const node = this.getTreeNode(parentLocation);
-		return node === this.root ? null : node.element;
+		return node.element;
 	}
 
-	getFirstElementChild(location: number[]): T | null {
+	getFirstElementChild(location: number[]): T | undefined {
 		const node = this.getTreeNode(location);
 
 		if (node.children.length === 0) {
-			return null;
+			return undefined;
 		}
 
 		return node.children[0].element;
 	}
 
-	getLastElementAncestor(location: number[]): T | null {
+	getLastElementAncestor(location: number[]): T | undefined {
 		const node = this.getTreeNode(location);
 
 		if (node.children.length === 0) {
-			return null;
+			return undefined;
 		}
 
 		return this._getLastElementAncestor(node);
 	}
 
-	private _getLastElementAncestor(node: ITreeNode<T, TFilterData>): T | null {
+	private _getLastElementAncestor(node: ITreeNode<T, TFilterData>): T {
 		if (node.children.length === 0) {
 			return node.element;
 		}
diff --git a/src/vs/base/browser/ui/tree/objectTree.ts b/src/vs/base/browser/ui/tree/objectTree.ts
index fcce8f4613f483fd756209194670b7bbafb7a207..90cedbbda8be6465bf53a45914e4321ab8b66444 100644
--- a/src/vs/base/browser/ui/tree/objectTree.ts
+++ b/src/vs/base/browser/ui/tree/objectTree.ts
@@ -32,11 +32,11 @@ export class ObjectTree<T extends NonNullable<any>, TFilterData = void> extends
 		children?: ISequence<ITreeElement<T>>,
 		onDidCreateNode?: (node: ITreeNode<T, TFilterData>) => void,
 		onDidDeleteNode?: (node: ITreeNode<T, TFilterData>) => void
-	): Iterator<ITreeElement<T>> {
+	): Iterator<ITreeElement<T | null>> {
 		return this.model.setChildren(element, children, onDidCreateNode, onDidDeleteNode);
 	}
 
-	protected createModel(view: ISpliceable<ITreeNode<T, TFilterData>>, options: IObjectTreeOptions<T, TFilterData>): ITreeModel<T, TFilterData, T> {
+	protected createModel(view: ISpliceable<ITreeNode<T, TFilterData>>, options: IObjectTreeOptions<T, TFilterData>): ITreeModel<T | null, TFilterData, T | null> {
 		return new ObjectTreeModel(view, options);
 	}
 }
\ No newline at end of file
diff --git a/src/vs/base/browser/ui/tree/objectTreeModel.ts b/src/vs/base/browser/ui/tree/objectTreeModel.ts
index 0aa4cedeb334b76c90a34273377b4e835a0a71cd..2d3ee5222cdafabf6dfe090a9833b1020eebfd64 100644
--- a/src/vs/base/browser/ui/tree/objectTreeModel.ts
+++ b/src/vs/base/browser/ui/tree/objectTreeModel.ts
@@ -11,10 +11,10 @@ import { ITreeModel, ITreeNode, ITreeElement } from 'vs/base/browser/ui/tree/tre
 
 export interface IObjectTreeModelOptions<T, TFilterData> extends IIndexTreeModelOptions<T, TFilterData> { }
 
-export class ObjectTreeModel<T extends NonNullable<any>, TFilterData = void> implements ITreeModel<T, TFilterData, T> {
+export class ObjectTreeModel<T extends NonNullable<any>, TFilterData extends NonNullable<any> = void> implements ITreeModel<T | null, TFilterData, T | null> {
 
-	private model: IndexTreeModel<T, TFilterData>;
-	private nodes = new Map<T, ITreeNode<T, TFilterData>>();
+	private model: IndexTreeModel<T | null, TFilterData>;
+	private nodes = new Map<T | null, ITreeNode<T, TFilterData>>();
 
 	readonly onDidChangeCollapseState: Event<ITreeNode<T, TFilterData>>;
 	readonly onDidChangeRenderNodeCount: Event<ITreeNode<T, TFilterData>>;
@@ -22,9 +22,9 @@ export class ObjectTreeModel<T extends NonNullable<any>, TFilterData = void> imp
 	get size(): number { return this.nodes.size; }
 
 	constructor(list: ISpliceable<ITreeNode<T, TFilterData>>, options: IObjectTreeModelOptions<T, TFilterData> = {}) {
-		this.model = new IndexTreeModel(list, options);
-		this.onDidChangeCollapseState = this.model.onDidChangeCollapseState;
-		this.onDidChangeRenderNodeCount = this.model.onDidChangeRenderNodeCount;
+		this.model = new IndexTreeModel(list, null, options);
+		this.onDidChangeCollapseState = this.model.onDidChangeCollapseState as Event<ITreeNode<T, TFilterData>>;
+		this.onDidChangeRenderNodeCount = this.model.onDidChangeRenderNodeCount as Event<ITreeNode<T, TFilterData>>;
 	}
 
 	setChildren(
@@ -32,9 +32,9 @@ export class ObjectTreeModel<T extends NonNullable<any>, TFilterData = void> imp
 		children: ISequence<ITreeElement<T>> | undefined,
 		onDidCreateNode?: (node: ITreeNode<T, TFilterData>) => void,
 		onDidDeleteNode?: (node: ITreeNode<T, TFilterData>) => void
-	): Iterator<ITreeElement<T>> {
+	): Iterator<ITreeElement<T | null>> {
 		const location = this.getElementLocation(element);
-		const insertedElements = new Set<T>();
+		const insertedElements = new Set<T | null>();
 
 		const _onDidCreateNode = (node: ITreeNode<T, TFilterData>) => {
 			insertedElements.add(node.element);
@@ -64,7 +64,7 @@ export class ObjectTreeModel<T extends NonNullable<any>, TFilterData = void> imp
 		);
 	}
 
-	private preserveCollapseState(elements: ISequence<ITreeElement<T>> | undefined): ISequence<ITreeElement<T>> {
+	private preserveCollapseState(elements: ISequence<ITreeElement<T | null>> | undefined): ISequence<ITreeElement<T | null>> {
 		const iterator = elements ? getSequenceIterator(elements) : Iterator.empty<ITreeElement<T>>();
 
 		return Iterator.map(iterator, treeElement => {
@@ -91,12 +91,12 @@ export class ObjectTreeModel<T extends NonNullable<any>, TFilterData = void> imp
 		return this.model.getParentElement(location);
 	}
 
-	getFirstElementChild(ref: T | null = null): T | null {
+	getFirstElementChild(ref: T | null = null): T | null | undefined {
 		const location = this.getElementLocation(ref);
 		return this.model.getFirstElementChild(location);
 	}
 
-	getLastElementAncestor(ref: T | null = null): T | null {
+	getLastElementAncestor(ref: T | null = null): T | null | undefined {
 		const location = this.getElementLocation(ref);
 		return this.model.getLastElementAncestor(location);
 	}
@@ -129,7 +129,7 @@ export class ObjectTreeModel<T extends NonNullable<any>, TFilterData = void> imp
 		this.model.refilter();
 	}
 
-	getNode(element: T | null = null): ITreeNode<T, TFilterData> {
+	getNode(element: T | null = null): ITreeNode<T | null, TFilterData> {
 		const location = this.getElementLocation(element);
 		return this.model.getNode(location);
 	}
diff --git a/src/vs/base/browser/ui/tree/tree.ts b/src/vs/base/browser/ui/tree/tree.ts
index e60d3794f3e85ad055d992263a77d34dfe1811f2..668386bff57ff0c6fad52ba99934c39350b773c8 100644
--- a/src/vs/base/browser/ui/tree/tree.ts
+++ b/src/vs/base/browser/ui/tree/tree.ts
@@ -92,11 +92,11 @@ export interface ITreeModel<T, TFilterData, TRef> {
 	getListIndex(location: TRef): number;
 	getNode(location?: TRef): ITreeNode<T, any>;
 	getNodeLocation(node: ITreeNode<T, any>): TRef;
-	getParentNodeLocation(location: TRef): TRef | null;
+	getParentNodeLocation(location: TRef): TRef;
 
-	getParentElement(location: TRef | null): T | null;
-	getFirstElementChild(location: TRef | null): T | null;
-	getLastElementAncestor(location: TRef | null): T | null;
+	getParentElement(location: TRef): T;
+	getFirstElementChild(location: TRef): T | undefined;
+	getLastElementAncestor(location: TRef): T | undefined;
 
 	isCollapsed(location: TRef): boolean;
 	setCollapsed(location: TRef, collapsed: boolean): boolean;
diff --git a/src/vs/base/test/browser/ui/tree/indexTreeModel.test.ts b/src/vs/base/test/browser/ui/tree/indexTreeModel.test.ts
index 07431d166b862a84a9abeab447f531b39215f1d7..25e84a34f4c673763a864a353ec6b0c857cc74d7 100644
--- a/src/vs/base/test/browser/ui/tree/indexTreeModel.test.ts
+++ b/src/vs/base/test/browser/ui/tree/indexTreeModel.test.ts
@@ -25,14 +25,14 @@ suite('IndexTreeModel', function () {
 
 	test('ctor', () => {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 		assert(model);
 		assert.equal(list.length, 0);
 	});
 
 	test('insert', () => {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{ element: 0 },
@@ -54,7 +54,7 @@ suite('IndexTreeModel', function () {
 
 	test('deep insert', function () {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -91,7 +91,7 @@ suite('IndexTreeModel', function () {
 
 	test('deep insert collapsed', function () {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -119,7 +119,7 @@ suite('IndexTreeModel', function () {
 
 	test('delete', () => {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{ element: 0 },
@@ -144,7 +144,7 @@ suite('IndexTreeModel', function () {
 
 	test('nested delete', function () {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -178,7 +178,7 @@ suite('IndexTreeModel', function () {
 
 	test('deep delete', function () {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -206,7 +206,7 @@ suite('IndexTreeModel', function () {
 
 	test('hidden delete', function () {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -231,7 +231,7 @@ suite('IndexTreeModel', function () {
 
 	test('collapse', () => {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -262,7 +262,7 @@ suite('IndexTreeModel', function () {
 
 	test('expand', () => {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -302,7 +302,7 @@ suite('IndexTreeModel', function () {
 
 	test('collapse should recursively adjust visible count', function () {
 		const list: ITreeNode<number>[] = [];
-		const model = new IndexTreeModel<number>(toSpliceable(list));
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -341,7 +341,7 @@ suite('IndexTreeModel', function () {
 			}
 		};
 
-		const model = new IndexTreeModel<number>(toSpliceable(list), { filter });
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1, { filter });
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -375,7 +375,7 @@ suite('IndexTreeModel', function () {
 			}
 		};
 
-		const model = new IndexTreeModel<number>(toSpliceable(list), { filter });
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1, { filter });
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -398,7 +398,7 @@ suite('IndexTreeModel', function () {
 			}
 		};
 
-		const model = new IndexTreeModel<number>(toSpliceable(list), { filter });
+		const model = new IndexTreeModel<number>(toSpliceable(list), -1, { filter });
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -437,7 +437,7 @@ suite('IndexTreeModel', function () {
 			}
 		};
 
-		const model = new IndexTreeModel<string>(toSpliceable(list), { filter });
+		const model = new IndexTreeModel<string>(toSpliceable(list), 'root', { filter });
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -483,7 +483,7 @@ suite('IndexTreeModel', function () {
 			}
 		};
 
-		const model = new IndexTreeModel<string>(toSpliceable(list), { filter });
+		const model = new IndexTreeModel<string>(toSpliceable(list), 'root', { filter });
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -529,7 +529,7 @@ suite('IndexTreeModel', function () {
 			}
 		};
 
-		const model = new IndexTreeModel<string>(toSpliceable(list), { filter });
+		const model = new IndexTreeModel<string>(toSpliceable(list), 'root', { filter });
 
 		model.splice([0], 0, Iterator.fromArray([
 			{
@@ -577,7 +577,7 @@ suite('IndexTreeModel', function () {
 
 		test('simple', function () {
 			const list: ITreeNode<number>[] = [];
-			const model = new IndexTreeModel<number>(toSpliceable(list));
+			const model = new IndexTreeModel<number>(toSpliceable(list), -1);
 
 			model.splice([0], 0, Iterator.fromArray([
 				{
@@ -607,7 +607,7 @@ suite('IndexTreeModel', function () {
 				}
 			};
 
-			const model = new IndexTreeModel<number>(toSpliceable(list), { filter });
+			const model = new IndexTreeModel<number>(toSpliceable(list), -1, { filter });
 
 			model.splice([0], 0, Iterator.fromArray([
 				{