#997 MKV muxer window can't be used if a first error occurs

[7heS4ge]

On MeGUI version 2944 :
In the MKV Muxer window, when we not enter a good path for the target file, there is an error (it's normal !). But if we attempt to change the path of the file, we can't create the job : the button Queue don't close the window. We must close MeGui entirely, relaunch MeGui and reopen the MKV Muxer window.