Crash in case of ambiguous grammar

[tirix]

See set.mm PR#2379:
In case of an ambiguous grammar ($a class A B : $.), there is a NullPointerException:

java.lang.NullPointerException
	at mmj.verify.LRParser.initialize(LRParser.java:175)
	at mmj.verify.LRParser.load(LRParser.java:320)
	at mmj.verify.LRParser.load(LRParser.java:317)
	at mmj.verify.LRParser.parseExpr(LRParser.java:382)
	at mmj.verify.Grammar.grammaticalParseSyntaxExpr(Grammar.java:732)
	at mmj.verify.GrammarAmbiguity.basicAmbiguityEdits(GrammarAmbiguity.java:208)
	at mmj.verify.Grammar.initializeGrammarTables(Grammar.java:1008)
	at mmj.verify.Grammar.initializeGrammar(Grammar.java:704)
	at mmj.util.GrammarBoss.initializeGrammar(GrammarBoss.java:288)
	at mmj.util.GrammarBoss.doParse(GrammarBoss.java:242)
	at mmj.util.Boss.lambda$0(Boss.java:107)
	at mmj.util.Boss.doRunParmCommand(Boss.java:121)
	at mmj.util.BatchFramework.executeRunParmCommand(BatchFramework.java:281)
	at mmj.util.BatchFramework.runIt(BatchFramework.java:223)
	at mmj.util.BatchMMJ2.main(BatchMMJ2.java:53)

It shall rather trigger a clean error complaining about the grammar being ambiguous.