fix strict null checks

src/vs/base/browser/ui/tree/dataTree.ts

@@ -118,8 +118,8 @@ export enum ChildrenResolutionReason {
 }
 
 export interface IChildrenResolutionEvent<T> {
-	element: T;
-	reason: ChildrenResolutionReason;
+	readonly element: T | null;
+	readonly reason: ChildrenResolutionReason;
 }
 
 export class DataTree<T extends NonNullable<any>, TFilterData = void> implements IDisposable {
@@ -134,7 +134,7 @@ export class DataTree<T extends NonNullable<any>, TFilterData = void> implements
 
 	get onDidChangeFocus(): Event<ITreeEvent<T>> { return mapEvent(this.tree.onDidChangeFocus, asTreeEvent); }
 	get onDidChangeSelection(): Event<ITreeEvent<T>> { return mapEvent(this.tree.onDidChangeSelection, asTreeEvent); }
-	get onDidChangeCollapseState(): Event<T> { return mapEvent(this.tree.onDidChangeCollapseState, e => e.element.element); }
+	get onDidChangeCollapseState(): Event<T> { return mapEvent(this.tree.onDidChangeCollapseState, e => e.element.element!); }
 
 	private _onDidResolveChildren = new Emitter<IChildrenResolutionEvent<T>>();
 	readonly onDidResolveChildren: Event<IChildrenResolutionEvent<T>> = this._onDidResolveChildren.event;
@@ -252,17 +252,21 @@ export class DataTree<T extends NonNullable<any>, TFilterData = void> implements
 		}
 	}
 
-	private setChildren(element: IDataTreeNode<T>, children?: ISequence<ITreeElement<IDataTreeNode<T>>>): void {
+	private setChildren(element: IDataTreeNode<T> | null, children?: ISequence<ITreeElement<IDataTreeNode<T>>>): void {
 		const insertedElements = new Set<T>();
 
 		const onDidCreateNode = (node: ITreeNode<IDataTreeNode<T>, TFilterData>) => {
-			insertedElements.add(node.element.element);
-			this.nodes.set(node.element.element, node.element);
+			if (node.element.element) {
+				insertedElements.add(node.element.element);
+				this.nodes.set(node.element.element, node.element);
+			}
 		};
 
 		const onDidDeleteNode = (node: ITreeNode<IDataTreeNode<T>, TFilterData>) => {
-			if (!insertedElements.has(node.element.element)) {
-				this.nodes.delete(node.element.element);
+			if (node.element.element) {
+				if (!insertedElements.has(node.element.element)) {
+					this.nodes.delete(node.element.element);
+				}
 			}
 		};