Merge pull request #86 from Unity-Technologies/siyaoH/1.17/fix2

fix gesture error

com.unity.uiwidgets/Runtime/gestures/multidrag.cs

 
             D.assert(_pointers.ContainsKey(pointer));
             GestureBinding.instance.pointerRouter.removeRoute(pointer, _handleEvent);
-            _pointers[pointer].dispose();
+            var pointerData = _pointers[pointer];
+            pointerData.dispose();
         }