diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index 55f5e2c88d677be1dec28af590a6467f7cbd18c3..d37bcefd81bb4c0ca6b540c2b75aa7645e3a55e7 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -42,7 +42,9 @@
 		"./vs/base/parts/tree/browser/treeDnd.ts",
 		"./vs/base/parts/tree/browser/treeModel.ts",
 		"./vs/base/parts/tree/browser/treeUtils.ts",
+		"./vs/base/parts/tree/browser/treeViewModel.ts",
 		"./vs/base/parts/tree/test/browser/treeModel.test.ts",
+		"./vs/base/parts/tree/test/browser/treeViewModel.test.ts",
 		"./vs/base/test/browser/browser.test.ts",
 		"./vs/base/test/browser/comparers.test.ts",
 		"./vs/base/test/browser/dom.test.ts",
@@ -868,4 +870,4 @@
 	"exclude": [
 		"./typings/require-monaco.d.ts"
 	]
-}
+}
\ No newline at end of file
diff --git a/src/vs/base/parts/tree/browser/treeViewModel.ts b/src/vs/base/parts/tree/browser/treeViewModel.ts
index 07f0c852e3a75b2110d1e6ee75af8c0ef32a77ab..823f0bb0a9d14420d5e28edb039980c1348607f2 100644
--- a/src/vs/base/parts/tree/browser/treeViewModel.ts
+++ b/src/vs/base/parts/tree/browser/treeViewModel.ts
@@ -15,21 +15,16 @@ export interface IViewItem {
 
 export class HeightMap {
 
-	private heightMap: IViewItem[];
-	private indexes: { [item: string]: number; };
+	private heightMap: IViewItem[] = [];
+	private indexes: { [item: string]: number; } = {};
 
-	constructor() {
-		this.heightMap = [];
-		this.indexes = {};
-	}
-
-	public getContentHeight(): number {
+	getContentHeight(): number {
 		let last = this.heightMap[this.heightMap.length - 1];
 		return !last ? 0 : last.top + last.height;
 	}
 
-	public onInsertItems(iterator: INextIterator<Item>, afterItemId: string | null = null): number {
-		let item: Item;
+	onInsertItems(iterator: INextIterator<Item>, afterItemId: string | null = null): number | undefined {
+		let item: Item | null = null;
 		let viewItem: IViewItem;
 		let i: number, j: number;
 		let totalSize: number;
@@ -82,16 +77,16 @@ export class HeightMap {
 		return sizeDiff;
 	}
 
-	public onInsertItem(item: IViewItem): void {
+	onInsertItem(item: IViewItem): void {
 		// noop
 	}
 
 	// Contiguous items
-	public onRemoveItems(iterator: INextIterator<string>): void {
-		let itemId: string;
+	onRemoveItems(iterator: INextIterator<string>): void {
+		let itemId: string | null = null;
 		let viewItem: IViewItem;
 		let startIndex: number | null = null;
-		let i: number;
+		let i = 0;
 		let sizeDiff = 0;
 
 		while (itemId = iterator.next()) {
@@ -112,7 +107,7 @@ export class HeightMap {
 			}
 		}
 
-		if (sizeDiff === 0) {
+		if (sizeDiff === 0 || startIndex === null) {
 			return;
 		}
 
@@ -126,18 +121,18 @@ export class HeightMap {
 		}
 	}
 
-	public onRemoveItem(item: IViewItem): void {
+	onRemoveItem(item: IViewItem): void {
 		// noop
 	}
 
-	public onRefreshItemSet(items: Item[]): void {
+	onRefreshItemSet(items: Item[]): void {
 		let sortedItems = items.sort((a, b) => this.indexes[a.id] - this.indexes[b.id]);
 		this.onRefreshItems(new ArrayIterator(sortedItems));
 	}
 
 	// Ordered, but not necessarily contiguous items
-	public onRefreshItems(iterator: INextIterator<Item>): void {
-		let item: Item;
+	onRefreshItems(iterator: INextIterator<Item>): void {
+		let item: Item | null = null;
 		let viewItem: IViewItem;
 		let newHeight: number;
 		let i: number, j: number | null = null;
@@ -171,19 +166,19 @@ export class HeightMap {
 		}
 	}
 
-	public onRefreshItem(item: IViewItem, needsRender: boolean = false): void {
+	onRefreshItem(item: IViewItem, needsRender: boolean = false): void {
 		// noop
 	}
 
-	public itemsCount(): number {
+	itemsCount(): number {
 		return this.heightMap.length;
 	}
 
-	public itemAt(position: number): string {
+	itemAt(position: number): string {
 		return this.heightMap[this.indexAt(position)].model.id;
 	}
 
-	public withItemsInRange(start: number, end: number, fn: (item: string) => void): void {
+	withItemsInRange(start: number, end: number, fn: (item: string) => void): void {
 		start = this.indexAt(start);
 		end = this.indexAt(end);
 		for (let i = start; i <= end; i++) {
@@ -191,7 +186,7 @@ export class HeightMap {
 		}
 	}
 
-	public indexAt(position: number): number {
+	indexAt(position: number): number {
 		let left = 0;
 		let right = this.heightMap.length;
 		let center: number;
@@ -217,15 +212,15 @@ export class HeightMap {
 		return this.heightMap.length;
 	}
 
-	public indexAfter(position: number): number {
+	indexAfter(position: number): number {
 		return Math.min(this.indexAt(position) + 1, this.heightMap.length);
 	}
 
-	public itemAtIndex(index: number): IViewItem {
+	itemAtIndex(index: number): IViewItem {
 		return this.heightMap[index];
 	}
 
-	public itemAfter(item: IViewItem): IViewItem {
+	itemAfter(item: IViewItem): IViewItem {
 		return this.heightMap[this.indexes[item.model.id] + 1] || null;
 	}
 
@@ -233,8 +228,8 @@ export class HeightMap {
 		throw new Error('not implemented');
 	}
 
-	public dispose(): void {
-		this.heightMap = null;
-		this.indexes = null;
+	dispose(): void {
+		this.heightMap = [];
+		this.indexes = {};
 	}
 }