Fix ss crash on game menu, fix ss on windowed mode

src/yuzu/main.cpp

@@ -753,7 +753,7 @@ void GMainWindow::InitializeHotkeys() {
             });
     connect(hotkey_registry.GetHotkey(main_window, QStringLiteral("Capture Screenshot"), this),
             &QShortcut::activated, this, [&] {
-                if (emu_thread->IsRunning()) {
+                if (emu_thread != nullptr && emu_thread->IsRunning()) {
                     OnCaptureScreenshot();
                 }
             });