Ignore removed/changed screens if no QIOSIntegration instance exists

QIOSTracker registers itself as handlers for system notifications about
changes of the screen environment. If no QIOSIntegration instance
exists, newly detected screens are not added to the list of known
screens (see screenConnected()). This, in turn, will result in a crash
if a screen is disconnected and removed in screenDisconnected() as it
is not known to qtPlatformScreenFor() and the function returns a
nullptr.

Consider the QIOSIntegration also whenever a screen is "changed". This
is more of a safety measure do avoid crashes for unknown screens.

This situation occurs if an iOS device is used to mirror the display
via AirPlay and no actual QGuiApplication exists, e.g. Qt is only
embedded in a Framework.

Pick-to: 6.5 6.2
Fixes: QTBUG-106701
Change-Id: Id778fc5afa7c284b0536ee02b1ba2c10321cc5b1
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Lars Schmertmann <lars.schmertmann@governikus.de>

src/plugins/platforms/ios/qiosscreen.mm

@@ -81,6 +81,9 @@ static QIOSScreen* qtPlatformScreenFor(UIScreen *uiScreen)
 
 + (void)screenDisconnected:(NSNotification*)notification
 {
+    if (!QIOSIntegration::instance())
+        return;
+
     QIOSScreen *screen = qtPlatformScreenFor([notification object]);
     Q_ASSERT_X(screen, Q_FUNC_INFO, "Screen disconnected that we didn't know about");
 
@@ -89,6 +92,9 @@ static QIOSScreen* qtPlatformScreenFor(UIScreen *uiScreen)
 
 + (void)screenModeChanged:(NSNotification*)notification
 {
+    if (!QIOSIntegration::instance())
+        return;
+
     QIOSScreen *screen = qtPlatformScreenFor([notification object]);
     Q_ASSERT_X(screen, Q_FUNC_INFO, "Screen changed that we didn't know about");