Merge bitcoin#12377: qt: Poll ShutdownTimer after init is done

2222bf0 qt: Poll ShutdownTimer after init is done (MarcoFalke)

Pull request description:

  The shutdown process has started in `requestShutdown`, but initialize will happily continue with `initializeResult` and start threads late in the shutdown progess. Deleting this running thread will crash the application according to the qt docs:
  https://github.com/qt/qtbase/blob/e5033a5c9b769815112e922d0b224af860afd219/src/corelib/thread/qthread.cpp#L412-L415

  Potential fix for bitcoin#12372 (comment)

  This reverts bitcoin#11831 for now and hopefully restores the previous behaviour.

Tree-SHA512: 8e1706afe90ddf2d972aca12c12d4cb2a9a4f38646c59c5466fe5a1a67361896b93c43917d5ac283841ee2bcc62e6bb8dc2bc81dea9129c899b354e9a4ef241b

src/qt/dash.cpp

@@ -424,7 +424,6 @@ void BitcoinApplication::createWindow(const NetworkStyle *networkStyle)
 
     pollShutdownTimer = new QTimer(window);
     connect(pollShutdownTimer, SIGNAL(timeout()), window, SLOT(detectShutdown()));
-    pollShutdownTimer->start(200);
 }
 
 void BitcoinApplication::createSplashScreen(const NetworkStyle *networkStyle)
@@ -552,6 +551,7 @@ void BitcoinApplication::initializeResult(bool success)
                          window, SLOT(message(QString,QString,unsigned int)));
         QTimer::singleShot(100, paymentServer, SLOT(uiReady()));
 #endif
+        pollShutdownTimer->start(200);
     } else {
         quit(); // Exit main loop
     }