title: Irony AST generation throws nullreference excepttion
body: <p>I'm getting started with Irony (version Irony_2012_03_15) but I pretty quickly got stuck when trying to generate an AST. Below is a completely strpped language that throws the exception:</p>
  
  <pre><code>[Language("myLang", "0.1", "Bla Bla")]
      public class MyLang: Grammar {
          public NModel()
              : base(false) {
  
              var number = TerminalFactory.CreateCSharpNumber("number");
              var binExpr = new NonTerminal("binExpr", typeof(BinaryOperationNode));
              var binOp = new NonTerminal("BinOp");
  
              binExpr.Rule = number + binOp + number;
              binOp.Rule = ToTerm("+");
              RegisterOperators(1, "+");
              //MarkTransient(binOp);
              this.Root = binExpr;
              this.LanguageFlags = Parsing.LanguageFlags.CreateAst; // if I uncomment this line it throws the error
          }
      } 
  </code></pre>
  
  <p>As soon as I uncomment the last line it throws a NullReferenceException in the grammar explorer or when i want to parse a test. The error is on AstBuilder.cs line 96:   </p>
  
  <pre><code>  parseNode.AstNode = config.DefaultNodeCreator();
  </code></pre>
  
  <p>DefaultNodeCreator is a delegate that has not been set. </p>
  
  <p>I've tried setting things with MarkTransient etc but no dice.</p>
  
  <p>Can someone help me afloat here? I'm proably missing something obvious. Looked for AST tutorials all over the webs but I can't seem to find an explanation on how that works. </p>
  
  <p>Thanks in advance, </p>
  
  <p>Gert-Jan</p>