closing trajectory model with coordset slider shown gives NameError: name 'session' is not defined

[Elaine Meng]

Closing the slider (clicking the "x") WITHOUT closing the trajectory also gives the error, but closing a trajectory for which the slider is not shown doesn't cause it.

Example:
open 1plx coord t
close

Error processing trigger "remove models"
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/core/triggerset.py", line 126, in invoke
return self._func(self._name, data)
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/commands/coordset_gui.py", line 56, in models_closed_cb
self.delete()
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/commands/coordset_gui.py", line 61, in delete
t = atomic.get_triggers(session)
NameError: name 'session' is not defined

[Tom Goddard]

Fixed.