Update Delete all non-Master layers.py

now 
also deletes empty layers befor delete other, but there is a new error-message i dont understand:

Traceback (most recent call last):
  File "<string>", line 38, in <module>
  File "<string>", line 21, in process
  File "/Folder/Glyphs/x_InstallDaten/Glyphs.app/Contents/Scripts/GlyphsApp.py", line 555, in __getitem__
    ExtraLayer = self._owner.pyobjc_instanceMethods.layers().objectAtIndex_(Index)
IndexError: NSRangeException - *** -[__NSArrayM objectAtIndex:]: index 4 beyond bounds [0 .. 3]