Fix possible null when populating calltree window

CHANGELOG.md

 #### Fixed
 
 * Fixed `OnValidate` for FirstPersonController leading to infinite import loop when displaying the inspector for a Prefab Asset.
+* Fix for null Callables in Callable Tree Window.
 
 ## 2019.3.0
 

Editor/CallTree/CallTreeWindow.cs

                             // Add Callables from this Callable[] array
                             foreach (var call in value)
                             {
+                                if (call != null)
                                 node.Children.Add(GetCallableNode(call, stack));
                             }
                         }