Merge pull request #6 from Unity-Technologies/fix_null_ref_bug

fix bug: potential null reference error

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

         const int mouseButtonNum = 3;
 
         void _handleViewMetricsChanged(string method, List<JSONNode> args) {
-            this._windowAdapter.onViewMetricsChanged();
-            this._displayMetrics.onViewMetricsChanged();
+            this._windowAdapter?.onViewMetricsChanged();
+            this._displayMetrics?.onViewMetricsChanged();
         }
 
         protected virtual void InitWindowAdapter() {