Add null window check

Author: ASalavei

Diff for: compose/ui/ui/src/uikitMain/kotlin/androidx/compose/ui/backhandler/UIKitBackGestureDispatcher.kt

@@ -94,13 +94,15 @@ internal class UIKitBackGestureDispatcher(
     fun onDidMoveToWindow(window: UIWindow?, composeRootView: UIView) {
         if (enableBackGesture) {
             removeGestureListeners()
-            var view: UIView = composeRootView
-            while (view.superview != window) {
-                view = requireNotNull(view.superview) {
-                    "Window is not null, but superview is null for ${view.debugDescription}"
+            if (window != null) {
+                var view: UIView = composeRootView
+                while (view.superview != window) {
+                    view = requireNotNull(view.superview) {
+                        "Window is not null, but superview is null for ${view.debugDescription}"
+                    }
                 }
+                addGestureListeners(view)
             }
-            addGestureListeners(view)
         }
     }