mainThreadWebview: activeEditor can be null

fixes #44546

src/vs/workbench/api/electron-browser/mainThreadWebview.ts

@@ -483,7 +483,7 @@ export class MainThreadWebviews implements MainThreadWebviewsShape {
 	private onEditorsChanged() {
 		const activeEditor = this._editorService.getActiveEditor();
 		let newActiveWebview: { input: WebviewInput, handle: WebviewHandle } | undefined = undefined;
-		if (activeEditor.input instanceof WebviewInput) {
+		if (activeEditor && activeEditor.input instanceof WebviewInput) {
 			for (const handle of map.keys(this._webviews)) {
 				const input = this._webviews.get(handle);
 				if (input.matches(activeEditor.input)) {
@@ -522,4 +522,4 @@ export class MainThreadWebviews implements MainThreadWebviewsShape {
 	WebviewEditor,
 	WebviewEditor.ID,
 	localize('webview.editor.label', "webview editor")),
-	[new SyncDescriptor(WebviewInput)]);
\ No newline at end of file
+	[new SyncDescriptor(WebviewInput)]);