iOS: Don't try to initialize a11y for views that have no platform window

The window may have been destroyed, but the UIView may still be
be referenced and kept alive by the a11y subsystem.

Pick-to: 6.2 5.15
Task-number: QTBUG-94530
Change-Id: I24f1c9d45e80c1bb4c92536e7f91533a94fd077f
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/platforms/ios/quiview_accessibility.mm

@@ -66,6 +66,11 @@
 
 - (void)initAccessibility
 {
+    // The window may have gone away, but with the view
+    // temporarily caught in the a11y subsystem.
+    if (!self.platformWindow)
+        return;
+
     static bool init = false;
     if (!init)
         QGuiApplicationPrivate::platformIntegration()->accessibility()->setActive(true);