[Configuration] NeuroCurator crashes if configuration window closed at launch

[pafonta]

NeuroCurator 0.4.5

Closing the configuration window at launch should not make NeuroCurator crash.

If this is the first launch, NeuroCurator could just close or starts with nothing loaded (maybe better for users who just want to take a look).

Steps:

1. neurocurator.
2. After the configuration window has been displayed, close it.

Stack trace:

Traceback (most recent call last):
  File "/Users/fonta/conda/envs/nc/lib/python3.7/site-packages/neurocurator/mainWin.py", line 173, in closeEvent
    window_settings = self.settings.config['WINDOW']
AttributeError: 'NoneType' object has no attribute 'config'
Abort trap: 6