William's explanation in msg123203 for the cause of the error and the solution for keeping a reference to "flist" is good. IDLE has only one instance of FileList while running anyways.

Attached is a patch that behaves like William's description.

The modification to PyShell.py is necessary for a corner-case. If the last IDLE window is closed with the Open File dialog still open, then FileList.py and WindowList.py both call .quit(). Without the modification, IDLE closes after selecting a file since root.mainloop() returns and root.destroy() gets executed. 

The alternative (and simpler) method is to have the Open File dialog be modal, but AFAIK it can't be set to modal.