Some SynthUI classes can request to create a SynthContext with null fields

public SynthContext constructor does not allow to use null arguments: @throws NullPointerException if component, region of style is null.

However, SynthLabelUI uses the SynthContext.getContext() method with null style.

Steps to reproduce:
- run a conditional breakpoint to SynthContext.getContext() method
- debug SwingSet2 application
- select Nimbus L&F

The SynthContext.getContext() method is called with null style.