This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Nicola, You need to look at the UserAction class. Since it is throwing the Exception in your class, you can debug it. Try looking at that exact line. If you can't figure it out, post the method in which the code is crashing.