NullPointerException on parse error

[lukaseder]

On certain parse errors, there is a null pointer exception being thrown. For instance, when parsing this:

abc = x [

The following exception is thrown:

java.lang.NullPointerException
	at chrriis.grammar.model.BNFToGrammar$Chunk.prune(BNFToGrammar.java:70)
	at chrriis.grammar.model.BNFToGrammar$Chunk.prune(BNFToGrammar.java:147)
	at chrriis.grammar.model.BNFToGrammar$Chunk.access$1(BNFToGrammar.java:68)
	at chrriis.grammar.model.BNFToGrammar.createRule(BNFToGrammar.java:330)
	at chrriis.grammar.model.BNFToGrammar.convert(BNFToGrammar.java:300)
	at chrriis.grammar.rrdiagram.RRDiagramTests.grammar(RRDiagramTests.java:95)
	at chrriis.grammar.rrdiagram.RRDiagramTests.testParseError(RRDiagramTests.java:78)

This can be rather cumbersome if the grammar is large, as there is no indication whatsoever about what went wrong.