Merge pull request #218 from Unity-Technologies/fix_null_wrapper

fix _wrapper null error

com.unity.uiwidgets/Runtime/engine/UIWidgetsPanel.cs

 
         void _handleViewMetricsChanged(string method, List<JSONNode> args) {
             using (Isolate.getScope(anyIsolate)) {
+                if (_wrapper == null) {
+                    return;
+                }
                 _wrapper.displayMetrics.onViewMetricsChanged();
                 Window.instance.updateSafeArea();
                 Window.instance.onMetricsChanged?.Invoke();