Fix DisplayMessage() not being visible when OBS is always on top

The previous behaviour would cause the impression of OSB being frozen
due to dialogs windows being opened behind OBS while they take over the
input focus.

src/utils/utility.cpp

@@ -510,7 +510,8 @@ bool DisplayMessage(const QString &msg, bool question, bool modal)
 		return (answer == QMessageBox::Yes);
 	} else if (question && modal) {
 		auto answer = QMessageBox::question(
-			nullptr,
+			static_cast<QMainWindow *>(
+				obs_frontend_get_main_window()),
 			obs_module_text("AdvSceneSwitcher.windowTitle"), msg,
 			QMessageBox::Yes | QMessageBox::No);
 		return answer == QMessageBox::Yes;