diff --git a/src/vs/workbench/browser/editor.ts b/src/vs/workbench/browser/editor.ts
index a7f3c099ea2458274b9147d848b1acf947397e30..5a7209fb2266780f24d37f9c18763c0ea26a7e29 100644
--- a/src/vs/workbench/browser/editor.ts
+++ b/src/vs/workbench/browser/editor.ts
@@ -34,14 +34,14 @@ export interface IEditorRegistry {
 	registerEditor(descriptor: IEditorDescriptor, editorInputDescriptor: SyncDescriptor<EditorInput>[]): void;
 
 	/**
-	 * Returns the editor descriptor for the given input or null if none.
+	 * Returns the editor descriptor for the given input or `undefined` if none.
 	 */
-	getEditor(input: EditorInput): IEditorDescriptor | null;
+	getEditor(input: EditorInput): IEditorDescriptor | undefined;
 
 	/**
 	 * Returns the editor descriptor for the given identifier or null if none.
 	 */
-	getEditorById(editorId: string): IEditorDescriptor | null;
+	getEditorById(editorId: string): IEditorDescriptor | undefined;
 
 	/**
 	 * Returns an array of registered editors known to the platform.
@@ -104,12 +104,12 @@ class EditorRegistry implements IEditorRegistry {
 		this.editors.push(descriptor);
 	}
 
-	getEditor(input: EditorInput): EditorDescriptor | null {
+	getEditor(input: EditorInput): EditorDescriptor | undefined {
 		const findEditorDescriptors = (input: EditorInput, byInstanceOf?: boolean): EditorDescriptor[] => {
 			const matchingDescriptors: EditorDescriptor[] = [];
 
 			for (const editor of this.editors) {
-				const inputDescriptors: SyncDescriptor<EditorInput>[] | undefined = this.mapEditorToInputs.get(editor);
+				const inputDescriptors = this.mapEditorToInputs.get(editor);
 				if (inputDescriptors) {
 					for (const inputDescriptor of inputDescriptors) {
 						const inputClass = inputDescriptor.ctor;
@@ -154,17 +154,17 @@ class EditorRegistry implements IEditorRegistry {
 			return descriptors[0];
 		}
 
-		return null;
+		return undefined;
 	}
 
-	getEditorById(editorId: string): EditorDescriptor | null {
+	getEditorById(editorId: string): EditorDescriptor | undefined {
 		for (const editor of this.editors) {
 			if (editor.getId() === editorId) {
 				return editor;
 			}
 		}
 
-		return null;
+		return undefined;
 	}
 
 	getEditors(): EditorDescriptor[] {