getEditor() return null instead of undefined

src/vs/workbench/browser/parts/editor/editorCommands.ts

@@ -443,11 +443,11 @@ export function splitEditor(editorGroupService: IEditorGroupsService, direction:
 	const newGroup = editorGroupService.addGroup(sourceGroup, direction);
 
 	// Split editor (if it can be split)
-	let editorToCopy: IEditorInput | null;
+	let editorToCopy: IEditorInput | undefined;
 	if (context && typeof context.editorIndex === 'number') {
 		editorToCopy = sourceGroup.getEditor(context.editorIndex);
 	} else {
-		editorToCopy = sourceGroup.activeEditor;
+		editorToCopy = types.withNullAsUndefined(sourceGroup.activeEditor);
 	}
 
 	if (editorToCopy && (editorToCopy as EditorInput).supportsSplitEditor()) {

src/vs/workbench/browser/parts/editor/editorGroupView.ts

@@ -729,7 +729,7 @@ export class EditorGroupView extends Themable implements IEditorGroupView {
 		return this.editors;
 	}
 
-	getEditor(index: number): EditorInput | null {
+	getEditor(index: number): EditorInput | undefined {
 		return this._group.getEditor(index);
 	}
 

src/vs/workbench/browser/parts/editor/tabsTitleControl.ts

@@ -695,7 +695,7 @@ export class TabsTitleControl extends TitleControl {
 
 	private updateDropFeedback(element: HTMLElement, isDND: boolean, index?: number): void {
 		const isTab = (typeof index === 'number');
-		const editor = typeof index === 'number' ? this.group.getEditor(index) : null;
+		const editor = typeof index === 'number' ? this.group.getEditor(index) : undefined;
 		const isActiveTab = isTab && !!editor && this.group.isActive(editor);
 
 		// Background

src/vs/workbench/common/editor/editorGroup.ts

@@ -138,16 +138,16 @@ export class EditorGroup extends Disposable {
 		return mru ? this.mru.slice(0) : this.editors.slice(0);
 	}
 
-	getEditor(index: number): EditorInput | null;
-	getEditor(resource: URI): EditorInput | null;
-	getEditor(arg1: number | URI): EditorInput | null {
+	getEditor(index: number): EditorInput | undefined;
+	getEditor(resource: URI): EditorInput | undefined;
+	getEditor(arg1: number | URI): EditorInput | undefined {
 		if (typeof arg1 === 'number') {
 			return this.editors[arg1];
 		}
 
 		const resource: URI = arg1;
 		if (!this.contains(resource)) {
-			return null; // fast check for resource opened or not
+			return undefined; // fast check for resource opened or not
 		}
 
 		for (const editor of this.editors) {
@@ -157,7 +157,7 @@ export class EditorGroup extends Disposable {
 			}
 		}
 
-		return null;
+		return undefined;
 	}
 
 	get activeEditor(): EditorInput | null {

src/vs/workbench/services/editor/common/editorGroupsService.ts

@@ -411,7 +411,7 @@ export interface IEditorGroup {
 	/**
 	 * Returns the editor at a specific index of the group.
 	 */
-	getEditor(index: number): IEditorInput | null;
+	getEditor(index: number): IEditorInput | undefined;
 
 	/**
 	 * Get all editors that are currently opened in the group optionally