Null in GetValidMethodInfo

I have this null reference exception in my pc build log file:

NullReferenceException: Object reference not set to an instance of an object at UnityEngine.Events.UnityEventBase.GetValidMethodInfo (System.Object obj, System.String functionName, System.Type[] argumentTypes) [0x00000] in :0

at UnityEngine.Events.UnityEventBase.FindMethod (System.String name, System.Object listener, PersistentListenerMode mode, System.Type argumentType) [0x00000] in :0

at UnityEngine.Events.UnityEventBase.FindMethod (UnityEngine.Events.PersistentCall call) [0x00000] in :0

at UnityEngine.Events.PersistentCall.GetRuntimeCall (UnityEngine.Events.UnityEventBase theEvent) [0x00000] in :0

at UnityEngine.Events.PersistentCallGroup.Initialize (UnityEngine.Events.InvokableCallList invokableList, UnityEngine.Events.UnityEventBase unityEventBase) [0x00000] in :0

at UnityEngine.Events.UnityEventBase.UnityEngine.ISerializationCallbackReceiver.OnAfterDeserialize () [0x00000] in :0

(Filename: Line: -1)

It seems fairly harmless as nothing is broken and it did not crash but what would cause this null to occur?