java.lang.NullPointerException in DefineGrammarItemsWalker.java:3896

[DoctorLai]

Here is the Sample.g

grammar Sample;

options {
    memoize=true;
    output=AST;
}

tokens {
    REGEXPTOKEN;
}

RegExpressionLiteral
    : 
    '/' (a=~('/' | NL))+ '/'
      -> ^(REGEXPTOKEN[$RegExpressionLiteral.start, $RegExpressionLiteral.text] $a)
     ;

NL: '\n' | '\r';

and here is the command java -jar antlr-3.5.2-complete.jar Sample.g

and here is the output:

error(10): internal error: Sample.g : java.lang.NullPointerException
org.antlr.grammar.v3.DefineGrammarItemsWalker.rewrite_atom(DefineGrammarItemsWalker.java:3896)
org.antlr.grammar.v3.DefineGrammarItemsWalker.rewrite_tree(DefineGrammarItemsWalker.java:3835)
org.antlr.grammar.v3.DefineGrammarItemsWalker.rewrite_element(DefineGrammarItemsWalker.java:3722)
org.antlr.grammar.v3.DefineGrammarItemsWalker.rewrite_alternative(DefineGrammarItemsWalker.java:3595)
org.antlr.grammar.v3.DefineGrammarItemsWalker.rewrite(DefineGrammarItemsWalker.java:3378)
org.antlr.grammar.v3.DefineGrammarItemsWalker.block(DefineGrammarItemsWalker.java:1643)
org.antlr.grammar.v3.DefineGrammarItemsWalker.rule(DefineGrammarItemsWalker.java:1254)
org.antlr.grammar.v3.DefineGrammarItemsWalker.rules(DefineGrammarItemsWalker.java:981)
org.antlr.grammar.v3.DefineGrammarItemsWalker.grammarSpec(DefineGrammarItemsWalker.java:572)
org.antlr.grammar.v3.DefineGrammarItemsWalker.grammar_(DefineGrammarItemsWalker.java:313)
org.antlr.tool.Grammar.defineGrammarSymbols(Grammar.java:791)
org.antlr.tool.CompositeGrammar.defineGrammarSymbols(CompositeGrammar.java:379)
org.antlr.Tool.process(Tool.java:484)
org.antlr.Tool.main(Tool.java:98)