iOS: Guard against displayLayer after platform window is gone

During shutdown the UIView might outlive its QIOSWindow platform
window. If we receive displayLayer calls, we need to bail out,
to avoid crashing when trying to send the expose event.

Pick-to: 6.6 6.5
Change-Id: I2f36a4dd825e91b348bc57a06e6aa6b1a12fd249
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit 483523ad9f9807e84ab6aa0f48712df0524dbe28)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/ios/quiview.mm

@@ -263,6 +263,9 @@ inline ulong getTimeStamp(UIEvent *event)
     Q_UNUSED(layer);
     Q_ASSERT(layer == self.layer);
 
+    if (!self.platformWindow)
+        return;
+
     [self sendUpdatedExposeEvent];
 }