gcylc: Fix bug in pop-up for tasks (soon to be) removed

[sadielbartholomew]

I have observed a minor bug in the GUI. When attempting to view the prereq's & outputs file from the View menu for task that is completed (naturally I discovered this when trying to access this file for a task that was on the verge of completing & must have completed before I hit the button), one gets either of the following tracebacks on master:

Traceback (most recent call last):
  File "${HOME}/cylc.git/lib/cylc/gui/app_gcylc.py", line 1633, in popup_requisites
    if not results or task_id in bad_items:
UnboundLocalError: local variable 'results' referenced before assignment

or:

Traceback (most recent call last):
  File "${HOME}/cylc.git/lib/cylc/gui/app_gcylc.py", line 1629, in popup_requisites
    results, bad_items = self.updater.client.get_info(
AttributeError: 'NoneType' object has no attribute 'get_info'

To recreate, right-click a task whilst running & keep the menu open until the task has completed & disappeared from the GUI (or is just about to), then click prereq's & outputs.

Suggested change seems to be sufficient, but for safety we might want to catch all exceptions from the try statement or initialise results & bad_items to empty strings?