Bugfix: "internal error" message shown when trying to step from a state with multiple target edges

[karoliineh]

When setting a breakpoint to the paper demo on line 21, and starting the debugger, stepping results in an "internal error". This is caused by targetEdge getting a null value, after which targetEdge.nodeId() is called, causing a NullPointerException. Without this proposed fix, the debugger will show a notification box with "Internal error" but will recover and step forward once a stepping button is pushed again.

The bug is probably caused by the fix from #74, where now the return statement return targetEdgesByCFGNode.size() == 1 ? targetEdgesByCFGNode.get(0) : null; of function findTargetEdge is reached (previously an exception was always thrown), and thus, the targetEdge in stepAllThreadsIntoMatchingEdge can get a null value.