#490 Error when trying to select File in Sound Acquisition

[Michael]

If you are using a brand new psfx file and don't have any file history, when you try to select a file in the Sound Acquisition dialog PAMGuard will throw a null pointer error. It does this because it is trying to load up the file chooser gui with the previously selected file, except that there is no file because there is no file history yet. Fixed by skipping that part if currFile==null