Right-click on cleared / empty table gives non-crashing error for `_check_all`

[marshallmcdonnell]

This doesn't crash the GUI or hinder operations so a low priority for now.

Traceback (most recent call last):
  File "/SNS/users/pf9/code/addie/addie/main.py", line 622, in table_right_click
    _o_table.right_click(position=position)
  File "/SNS/users/pf9/code/addie/build/lib/addie/processing/idl/table_handler.py", line 183, in right_click
    elif action == _check_all:
UnboundLocalError: local variable '_check_all' referenced before assignment