On attempt to open an XTA file without a matching UGI file, the GUI generates a FileNotFoundException followed by a NullPointerException. The XTA file is not opened.
It seems the problem is actually much worse: It has nothing to do with missing UGI files. We simply cannot read TA and XTA files in the GUI in alpha 2. Also, the missing UGI error message is never shown, even when the UGI file is missing. Instead an error message is printed to stderr. This part is a minor issue though, as this error message has been eliminated in later version of the GUI.
A fix is already in CVS (revision 1300 seems to fix the problem).