java.lang.NullPointerException in DefineGrammarItemsWalker.java:3896

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)

Reproduce the failure with the supplied Sample.g and the java -jar antlr-3.5.2-complete.jar Sample.g command. Start at org.antlr.grammar.v3.DefineGrammarItemsWalker.rewrite_atom in DefineGrammarItemsWalker.java:3896, then trace the callers shown in the stack trace. Done means this grammar no longer produces an internal NullPointerException.

Written by the indexing model from the issue text.