NPE with negative conditions

[fdouglis]

I tried creating a domain using negative conditions. After getting past the lex error due to failing to include the spec, I hit a null pointer exception:

	at fr.uga.pddl4j.problem.FinalizedProblem.extractRelevantFluents(FinalizedProblem.java:373)
	at fr.uga.pddl4j.problem.DefaultProblem.finalization(DefaultProblem.java:299)
...```

If I add a system print to just before this, it is clear it is tripping over  `(not (6 4 2))`.  It gets a null `exp.getSymbol()`.  I tried a band-aid: only insert to to `this.numericFluents` if non-null, but that has other cascaded errors because the length of the fluents no longer is as expected.  

Ring any bells?  

I can post an example, but first I would need to sanitize it somehow, so first I want to see if this is a known issue.