error adding distance if Distances window previously closed

[Elaine Meng]

I had a few distances, showed the Distance tool, and then deleted all the distances and closed the tool. Then later in that session I added a distance with the selection context menu. Distance is shown OK in the graphics window but there is a traceback

distance /A:64@O /B:68@ODistance between /A HOH 64 O and /B HOH 68 O: 4.800Å
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 130, 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/dist_ui/tool.py", line 144, in _fill_table

update = len(pbs) == self.table.rowCount()

RuntimeError: wrapped C/C++ object of type QTableWidget has been deleted

Error processing trigger "update": wrapped C/C++ object of type QTableWidget has been deleted:
RuntimeError: wrapped C/C++ object of type QTableWidget has been deleted

File "/Users/chimera/Applications/ChimeraX_Daily.app/Contents/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/chimerax/dist_ui/tool.py", line 144, in _fill_table
update = len(pbs) == self.table.rowCount()

See log for complete Python traceback.

[Eric Pettersen]

Wasn't deregistering all trigger handlers when tool deleted