gcylc: fix led treeview error

I've noticed that you can sometimes get a traceback when closing the LED view in gcylc:
```
Traceback (most recent call last):
  File "......................./lib/cylc/gui/stateview.py", line 624, in update_gui
    self.ledview_widgets()
  File "...................../lib/cylc/gui/stateview.py", line 540, in ledview_widgets
    self.led_treeview.get_selection().set_mode( gtk.SELECTION_NONE )
AttributeError: 'NoneType' object has no attribute 'set_mode'
```

This seems to happen for very large suites when closing an ungrouped LED view. This change fixes it.

@hjoliver, please review.