Update custom_layout.cs

com.unity.uiwidgets/Runtime/rendering/custom_layout.cs

     }
 
     public abstract class MultiChildLayoutDelegate {
-        MultiChildLayoutDelegate(Listenable relayout = null) {
+        protected MultiChildLayoutDelegate(Listenable relayout = null) {
             _relayout = relayout;
         }
         public readonly Listenable _relayout;