Unfortunately it is hard to say what exactly may be incorrect.
It seems that this exception is popular in Google.
Well, assuming this is commons-digester 1.3,
reader = getParser().getXMLReader();
So the problem is that the digester hasn't been initialized with a
parser. Not really sure what would cause that, but it's probably
outside of the scope of facelets or JSF.
2. Or this one.http://www.coderanch.com/t/423990/XML-R ... -Exception
I found the solution. It is because of library files, they were got corrupted some how! i copied them and everything is fine.
So, it seems that this problem is unrelated with WindowBuilder itself.
Can you reinstall JDK, may be something is broken?