Throw exception in an impossible null case

src/main/java/org/sablecc/sablecc/GenerateAlternativeCodeForParser.java

@@ -214,8 +214,7 @@ public class GenerateAlternativeCodeForParser extends DepthFirstAdapter
       }
       else if(type_name.equals("null"))
       {
-        type_name = "null";
-        type = "Object";
+        throw new RuntimeException("Simple term has null type - this should never happen");
       }
       else
       {