'DummySimulator' object has no attribute 'closed'

[jgosmann]

This exception is printed on the command line if you create a Simulator within the GUI:

Exception ignored in: <bound method Simulator.__del__ of <nengo_gui.exec_env.make_dummy.<locals>.DummySimulator object at 0x7f8dc20b4e10>>
Traceback (most recent call last):
  File "/home/jgosmann/.pyenv/versions/3.5.3/lib/python3.5/site-packages/nengo/simulator.py", line 180, in __del__
    if not self.closed:
AttributeError: 'DummySimulator' object has no attribute 'closed'

[arvoelke]

@VictorBoutin also just saw this but for a different reason. Not sure how he got into this state (something about adding a CircularConvolution network), but he didn't have a simulator within the GUI, and he had to restart the server to fix the issue.