prevent use after free through focusOutEvent when window is closed

src/frontend/qt_sdl/Window.cpp

@@ -780,6 +780,10 @@ void MainWindow::closeEvent(QCloseEvent* event)
     Config::Save();
 
     emuInstance->deleteWindow(windowID, false);
+
+    // emuInstance may be deleted
+    // prevent use after free from us
+    emuInstance = nullptr;
     QMainWindow::closeEvent(event);
 }
 
@@ -970,7 +974,10 @@ void MainWindow::focusInEvent(QFocusEvent* event)
 
 void MainWindow::focusOutEvent(QFocusEvent* event)
 {
-    emuInstance->audioMute();
+    // focusOutEvent is called through the window close event handler
+    // prevent use after free
+    if (emuInstance)
+        emuInstance->audioMute();
 }
 
 void MainWindow::onAppStateChanged(Qt::ApplicationState state)