Crash with NullPointerException in SymbolChecks.checkForModeConflicts with empty lexer

[octylFractal]

If I provide an empty lexer, then ANTLR will crash with a confusing NullPointerException. I don't think an empty lexer is really a common case, but the NPE had me worried for a little bit because I didn't understand what was causing it.

The cause is that LexerGrammar doesn't initialize modes until a rule is added.

Relevant stacktrace:

Caused by: java.lang.NullPointerException
    at org.antlr.v4.semantics.SymbolChecks.checkForModeConflicts(SymbolChecks.java:278)
    at org.antlr.v4.semantics.SemanticPipeline.process(SemanticPipeline.java:132)
    at org.antlr.v4.Tool.processNonCombinedGrammar(Tool.java:399)
    at org.antlr.v4.Tool.process(Tool.java:386)
    at org.antlr.v4.Tool.processGrammarsOnCommandLine(Tool.java:345)