tree: more type cleanup

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

@@ -296,7 +296,7 @@ export abstract class AbstractTree<T, TFilterData, TRef> implements IDisposable
 		return this.model.getFirstElementChild(location);
 	}
 
-	getLastElementAncestor(location: TRef): T | undefined {
+	getLastElementAncestor(location?: TRef): T | undefined {
 		return this.model.getLastElementAncestor(location);
 	}
 

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

@@ -443,7 +443,7 @@ export class IndexTreeModel<T extends Exclude<any, undefined>, TFilterData = voi
 		return node.children[0].element;
 	}
 
-	getLastElementAncestor(location: number[]): T | undefined {
+	getLastElementAncestor(location: number[] = []): T | undefined {
 		const node = this.getTreeNode(location);
 
 		if (node.children.length === 0) {

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

@@ -96,7 +96,7 @@ export interface ITreeModel<T, TFilterData, TRef> {
 
 	getParentElement(location: TRef): T;
 	getFirstElementChild(location: TRef): T | undefined;
-	getLastElementAncestor(location: TRef): T | undefined;
+	getLastElementAncestor(location?: TRef): T | undefined;
 
 	isCollapsed(location: TRef): boolean;
 	setCollapsed(location: TRef, collapsed: boolean): boolean;