on_exit function fix if self.file is missing
on_exit function fix if self.file is missing

HarmtH/vim-airline,dalguji/vim-airline,chrisbra/vim-airline,tschundler/dotfiles,yzhou61/vim-airline,bling/vim-airline,unphased/vim-airline,rchicoli/vim-airline,tschundler/dotfiles,tschundler/dotfiles