fix cupertino button warning

Runtime/cupertino/button.cs

 using System;
+using RSG;
 using Unity.UIWidgets.animation;
 using Unity.UIWidgets.foundation;
 using Unity.UIWidgets.gestures;
                 ? this._animationController.animateTo(1.0f, duration: kFadeOutDuration)
                 : this._animationController.animateTo(0.0f, duration: kFadeInDuration);
 
-            ticker.Then<VoidCallback>(() => {
+            ticker.Then(() => {
-
-                return null;
             });
         }
 

Samples/UIWidgetsGallery/demo/cupertino/cupertino_navigation_demo.cs.meta

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