fix null ptr

com.unity.uiwidgets/Runtime/codegen/TileContainerRenderObjectMixin.mixin.gen.cs

         return true;
     }
 
-    public ChildType this[int index]
-    {
-        get => _childRenderObjects[index];
+    public ChildType this[int index] {
+        get { return _childRenderObjects.ContainsKey(index) ? _childRenderObjects[index] : null; }
         set
 
         {