Program exits upon calling Gtk.File_Chooser_Dialog

Program exits upon calling Gtk.File_Chooser_Dialog

[hreba]

I am using GNAT-2016 on linux mint.

When I try to open a GTK file chooser dialog, e.g. calling

    Gtkada.File_Selection.File_Selection_Dialog

the program exits with the message

    (sos:3475): GLib-GIO-ERROR **: Settings schema
    'org.gtk.Settings.FileChooser' does not contain a key named
    'sort-directories-first'
    Trace/breakpoint trap

The same thing happens even when I call emacs from within my program with

    GNAT.Expect.Non_Blocking_Spawn (fd, "xdg-open", args);

and then click on the "Save As" menu item in the emacs window.

Searching the net I found that the reason is a buggy installation, where 
multiple versions of the settings scheme exist, and that the system 
variable "XDG_DATA_DIRS" determines which is used. Searching for 
"org.gtk.Settings.FileChooser.gschema.xml" got me 4 hits, 2 of them 
containing the key "sort-directories-first".

- Changing "XDG_DATA_DIRS" doesn't help because it is set automatically
   during boot.
- Replacing the 2 files with the missing key with one that contains it
   didn't help either.

So I think I have to repair my gtkada installation, I just don't know how.
-- 
Frank Hrebabetzky		+49 / 6355 / 989 5070