مشکل در ارور

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.Edge.WakeUp () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Edge.cs:114)
UnityEditor.Graphs.Graph.DoWakeUpEdges (System.Collections.Generic.List`1[T] inEdges, System.Collections.Generic.List`1[T] ok, System.Collections.Generic.List`1[T] error, System.Boolean inEdgesUsedToBeValid) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:387)
UnityEditor.Graphs.Graph.WakeUpEdges (System.Boolean clearSlotEdges) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:286)
UnityEditor.Graphs.Graph.WakeUp (System.Boolean force) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:272)
UnityEditor.Graphs.Graph.WakeUp () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:250)
UnityEditor.Graphs.Graph.OnEnable () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Graph.cs:245)

سلام ببخشید من تا قسمت 15 اموزش طبق فیلم شما رفتم ولی با این اخطار مواجه شدم اگه کمک کنید خیلی ممنون میشم