It would appear that it's illegal to call gtk_level_bar_set_value() with
a value of 0. Or, and that's just as likely, I don't understand how a
GtkLevelBar is supposed to work. You don't even have to call
gtk_level_bar_set_value() at all, since 0 is the default value of such a
bar.
All of this doesn't really matter, though, since GtkEntry has a built-in
progress bar that we can use.