Please Help!! NullReferenceException when clicking restore default pose

Hello Guys! I am new to Unity Engine, I want to ask about this error lately. I already made my first character and some animation on it. This one was bothering me when i click in hierarcy and click my game object (i name it as player), then "Restore a default pose" on the inspector menu. There was a notification that Object reference was not set to an instance, But the animation was worked perfectly. Is it will be a trouble later or not? Thanks guys! (sorry for my bad english)

NullReferenceException: Object reference not set to an instance of an object UnityEditor.Experimental.U2D.IK.IKManager2DEditor.DoRestoreDefaultPoseButton () (at Library/PackageCache/com.unity.2d.ik@1.0.8-preview.1/Editor/Inspectors/IKManager2DEditor.cs:156) UnityEditor.Experimental.U2D.IK.IKManager2DEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.2d.ik@1.0.8-preview.1/Editor/Inspectors/IKManager2DEditor.cs:136) UnityEditor.InspectorWindow.DoOnInspectorGUI (System.Boolean rebuildOptimizedGUIBlock, UnityEditor.Editor editor, System.Boolean wasVisible, UnityEngine.Rect& contentRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1625) UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)