[gtkada] Add_From_File with null for Error.

[Emmanuel Briot]

> As in C GTK programming, I put null for Error parameter thus I got:
> Exception name: CONSTRAINT_ERROR
> Message: gtk-builder.adb:149 access check failed

This is an error in gtk+ itself, or rather in the .gir file they export.
Your patch is incorrect since it manually modified a file that is generated
automatically.

I have put a workaround in GtkAda, but the recommendation is not to
pass null for the error, you really should not be ignoring errors in the
first place.

regards
Emmanuel