Custom tree walkers have freedom in modifying the AST but when you try to add a new query component (i.e. new join in walkSelectStatement() ) to the AST then the SqlWalker throws an exception because it does not has the new component in its _queryComponents array. I see two possible ways to resolve this:
1. Modify the Parser class in order to allow tree walkers to modify queryComponents and pass changed queryComponents to the SqlWalker
2. Improve SqlWalker so it can extract and prepare needed information about queryComponent based on AST when it does not have them.