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.
Plain and simple: something is null. The stack trace would go along way toward pinning this down. If the top frame is in this file, on this line, then "method" is null. If it's an InvocationTargetException and the contained exception is null, then one of the arguments is probably null and shouldn't be. If the top frame is in invoke() itself, then one of the two arguments to invoke() is likely null, or your passing a "null" for a primitive type in "argArray".
If you can't puzzle it out this way, putting in a bunch of print statements to print your variables would be a help.