Editor-script works however i get an error message "DockArea.EndOffsetArea()

Hi, i´ve created an Editor-script which creates a whole folder of geometry files. It´s works great and the assetbundle is correct. The problem is the upcoming error message in the console:

NullReferenceException: Object reference not set to an instance of an object

UnityEditor.DockArea.EndOffsetArea ()

UnityEditor.DockArea.OnGUI ()

Any reasons for that? Any ideas? Thanks for your help....