change Exception to NPE
change var name for better understanding
replaced unused code eith TODO
replace comment
set graph to null, after leaving start-method
added testclass for guistarter
Merge branch 'gui' of https://github.com/schauder/degraph.git into gui
removed one test, fixed the other one

The two tests in the comments had two issues:

1. when calling *launch* which is called inside *show* *start* gets called via reflection. Any exception thrown by *start* gets wrapped and therefore not detected by JUnit as expected.

2. when calling *launch* in the second test, the Application class complains that it must not get called twice.