Fixed two NullPointerException when Context.hashcode() is called.

[rondicus]

In both cases Context.getInstance() was called with a null-graph, which causes a NPE a bit further along when context.hashcode() is called.

I was thinking about a UnitTest, but the expection happens during the AWT-Paint phase, so i scrapped that idea for now.
The Problem is observable in one of the demos: EdgeLabelDemo

[rondicus]

Also thanks a lot for maintaining the project 👍

[jrtom]

Thanks for the catch. Mind adding a Preconditions.checkNotNull() to EdgeIndexFunction.getIndex()?

(At that point a unit test becomes less obviously necessary.)

[rondicus]

I was away for a few days but added it now

[jrtom]

Thanks!