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…

Like i said in previous post, the solution is to call "EditorGUIUtility.ExitGUI();".