Glad you figured it out. If you go to options>console you can bring up the console to see errors thrown (not sure if you knew this but I figured you might not have known if you posted here before finding the NPE!). Alternatively you can boot from cmd and have it print there, using System.out.println.
In terms of walking, i'd suggest against using the webwalker liberally. As a general rule of thumb, the only occasion you should warrant using the webwalker is when you know where you are going but not where you are walking from. Use paths for the rest!
Best
Apa