second "select" command of any session gives traceback

[Elaine Meng]

Seems like no matter what the details of the "select" command, the second one used per session gives an error. For example,

open 2gbp
sel ions
sel ligand

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 188, 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 2499, 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/core/commands/select.py", line 36, in select
clear_selection(session, "select_clear", undo_state)
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/commands/select.py", line 153, in clear_selection
session.selection.undo_add_selected(session, undo_state, False)
File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/core/selection.py", line 79, in undo_add_selected
undo_state.add(a, "selected", orig, new_state)
AttributeError: 'Session' object has no attribute 'add'

Left unassigned because I didn't know who should be assigned.

[Conrad Huang]

Same as #871