AnimationControllerのStateが何故か読み込めなくなるエラー

急にAnimationControllerのStateが何故か読み込めなくなるエラーがありました．
<img width="321" alt="99526267011e34c0cf4c92391440ed9b.png" src="https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/156186/d9c10934-2253-63a7-45ad-0195d613d7fb.png">
Consoleを見るとこんなエラーが出てました．
`
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Graphs.AnimationStateMachine.Graph.GenerateConnectionKey (UnityEditor.Graphs.Node srcNode, UnityEditor.Graphs.Node dstNode) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/Graph.cs:264)
UnityEditor.Graphs.AnimationStateMachine.Graph.GetEdgeInfo (UnityEditor.Graphs.Edge edge) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/Graph.cs:283)
UnityEditor.Graphs.AnimationStateMachine.EdgeGUI.DoEdges () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/EdgeGUI.cs:71)
UnityEditor.Graphs.AnimationStateMachine.GraphGUI.OnGraphGUI () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/AnimationStateMachine/GraphGUI.cs:390)
UnityEditor.Graphs.AnimatorControllerTool.StateMachineView (UnityEngine.Rect position, System.Single zoomLevel) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:1888)
UnityEditor.Graphs.AnimatorControllerTool.DoGraph (UnityEngine.Rect graphRect, System.Single zoomLevel) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:1251)
UnityEditor.Graphs.AnimatorControllerTool.<SetupGUI>m__D () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:1663)
UnityEditor.Graphs.AnimatorControllerTool.ScopedOnGUI (System.Action onGUIHandler) (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:1423)
UnityEditor.Graphs.AnimatorControllerTool.<SetupGUI>m__5 () (at C:/buildslave/unity/build/Editor/Graphs/UnityEditor.Graphs/Animation/AnimatorControllerTool.cs:1660)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:266)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:438)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleIMGUIEvent () (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:412)
UnityEngine.Experimental.UIElements.ImmediateStylePainter.DrawImmediate (System.Action callback) (at C:/buildslave/unity/build/Modules/UIElements/ImmediateStylePainter.cs:113)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoRepaint (UnityEngine.Experimental.UIElements.IStylePainter painter) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:98)
UnityEngine.Experimental.UIElements.VisualElement.Repaint (UnityEngine.Experimental.UIElements.IStylePainter painter) (at C:/buildslave/unity/build/Modules/UIElements/VisualElement.cs:862)
……
`

AnimationStateMachine NullReferenceExceptionで調べてみるとこんな記事が
https://answers.unity.com/questions/1020334/unexplanable-null-refrence-exception.html
結構前から時々起きるエラーみたいですね…．再起動したら直ってStateが全部表示されました．