"label del" causes traceback

[Elaine Meng]

It doesn't seem to matter which kind of 3D labels (atom, residue, pseudobond), any case I tried followed by "label delete" gives

Traceback (most recent call last):
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/cmd_line/tool.py", line 177, in execute
cmd.run(cmd_text)
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/commands/cli.py", line 2397, in run
result = ci.function(session, kw_args)
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/label/label3d.py", line 117, in label_delete
session.models.close([lm])
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/models.py", line 398, in close
self.remove(models)
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/models.py", line 376, in remove
mlist.sort(key=lambda m: len(m.id), reverse=True)
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/models.py", line 376, in <lambda>
mlist.sort(key=lambda m: len(m.id), reverse=True)
TypeError: object of type 'NoneType' has no len()

[Tom Goddard]

Fixed.

Not sure how I missed this in testing. I guess because I didn't delete all labels, and it requires deleting all labels for a model to et this error.