Right clicking while building tags causes error

[stephenfin]

Right clicking while building tags causes the following error:

Traceback (most recent call last):
  File "C:\Program Files\Sublime Text 3\sublime_plugin.py", line 463, in is_visible_
    raise ValueError("is_visible must return a bool", self)
ValueError: ('is_visible must return a bool', <FileDiffs.file_diffs.FileDiffPreviousCommand object at 0x0000000006674F60>)

Clearly one of the "mouse handlers" is causing issues.