ScriptableObject.ctor is not allowed to be called from a ScriptableObject constructor (or instance field initializer), call it in OnEnable instead. Called from ScriptableObject 'ArtToolWindow'.
See "Script Serialization" page in the Unity Manual for further details.
UnityEditor.Editor:.ctor()
PlayAnimation:.ctor() (at Assets/Editor/ArtTools/PlayAnimation.cs:20)
ArtToolWindow:.ctor() (at Assets/Editor/ArtTools/ArtToolWindow.cs:19)