[Qt] if the capture is stopped before we received any packet...

... we go back to the main welcome screen like we do in GTK

Change-Id: I64ef29665af61da55c1971ca59d1fab25d205874
Reviewed-on: https://code.wireshark.org/review/9071
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>

ui/qt/main_window_slots.cpp

@@ -686,6 +686,8 @@ void MainWindow::captureFileClosed() {
 
     main_ui_->statusBar->popFileStatus();
 
+    main_ui_->mainStack->setCurrentWidget(main_welcome_);
+
     setTitlebarForSelectedTreeRow();
     setMenusForSelectedTreeRow();
 }