Added a verification to avoid the error while quitting the app.

packages/app-desktop/ElectronAppWrapper.ts

@@ -174,7 +174,7 @@ export default class ElectronAppWrapper {
 					// so that it can tell us if we can really close the app or not.
 					// Search for "appClose" event for closing logic on renderer side.
 					event.preventDefault();
-					this.win_.webContents.send('appClose');
+					if (this.win_) this.win_.webContents.send('appClose');
 				} else {
 					// If the renderer process has responded, check if we can close or not
 					if (this.rendererProcessQuitReply_.canClose) {