GraphEditor : Exception then crash when root's parent is a deleted node

[themissingcow]

0.54.2.1

1. Make sure that your Gaffer session contains two Graph Editor panels.
2. Create a box in Graph Editor 1.
3. Go inside the box in Graph Editor 1
4. Create another box and go inside it (in Graph Editor 1). Your tab bar should now read Graph Editor:Box/Box.
5. Delete the top-most box in Graph Editor 2.
6. Press Arrow+Up to go outside the box in Graph Editor 1.
7. Exception error:

Traceback (most recent call last):
  File "/packages/vendor/ImageEngine/gaffer/linux_64/0.55.0.0/python/GafferUI/Widget.py", line 945, in eventFilter
    return self.__keyPress( qObject, qEvent )
  File "/packages/vendor/ImageEngine/gaffer/linux_64/0.55.0.0/python/GafferUI/Widget.py", line 1020, in __keyPress
    return widget._keyPressSignal( widget, event )
IECore.Exception: Object "ScriptNode" is not an ancestor of "Box".

9. If you press the Arrow + Up a second time in Graph Editor Fix type name clash #1, you will generate a segmentation fault:

****
* Arnold 5.4.0.1 [197d4d5e] linux clang-5.0.0 oiio-2.1.0 osl-1.11.0 vdb-4.0.0 clm-1.0.3.513 rlm-12.4.2 optix-6.0.0 2019/09/12 15:05:20
* CRASHED in Gaffer::GraphComponent::signals at 00:00:00
* signal caught: SIGSEGV -- Invalid memory reference (address not mapped to object)```