iOS: Guard traitCollectionDidChange against lack of QGuiApplication

During shutdown we may get traitCollectionDidChange for our QUIWindow,
as it may outlive the QGuiApplication and the QWindows due to hanging
around in an auto-release pool.

Pick-to: 6.5
Change-Id: Ifc6471b933fb1b4d1e83e9b734aac31044dce03e
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit 4dbc267)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 8543be7)

src/plugins/platforms/ios/qiosscreen.mm

@@ -184,6 +184,9 @@ - (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection
 {
     [super traitCollectionDidChange:previousTraitCollection];
 
+    if (!qGuiApp)
+        return;
+
     Qt::ColorScheme colorScheme = self.traitCollection.userInterfaceStyle
                               == UIUserInterfaceStyleDark
                               ? Qt::ColorScheme::Dark