Unable to create Pack: AttributeError: 'NoneType' object has no attribute 'addObject'

[bosd]

Using freecad 0.20
I'm unable to create a pack.
After seleccting a cell, filling the parameters I get this error.

22:50:50 Traceback (most recent call last):
22:50:50 File "/home/user/.var/app/org.freecadweb.FreeCAD/data/FreeCAD/Mod/battery_pack/batteryPackDialog.py", line 79, in createPack
22:50:50 self.part = App.activeDocument().addObject('App::Part',generate_freecad_name())
22:50:50 AttributeError: 'NoneType' object has no attribute 'addObject'