Have a couple of songs loaded, then delete(or move) one of them from the file system. Now if you play the respective song, the program will crash.
Here even renaming or deleting the file does not hear the crash program, but was no music, the buttons did not respond and just select another song in the list to resolve, so I made a patch to see if the song exists before start an music, failing that, it to the current song and displays the title of the player: "KISS Player | music not found - Resync music folder" and after that the folders are synchronized again.
Instead of displaying that message in the window title, stop the song and start syncing, we should just skip the song. Maybe we could highlight this song with a strikethrough... what do you think?
In attachment I've prepared a small patch. Check the wiki to see how to apply.
Also, try to use FLTK functions because they handle Unicode transparently: