Created attachment 252961 [details] [review]
patch to solve the issue

From Fedora bug report https://bugzilla.redhat.com/show_bug.cgi?id=1000559:

When using the find-duplicates extension and selecting a folder using pre-defined bookmarks like "Desktop", gthumb crashes.

How to reproduce:
- Edit -> Find Duplicates
- Select e.g. "Desktop" with the gtkfilechooserbutton
- Find

Current result:
Program received signal SIGSEGV, Segmentation fault.
gth_file_source_set_cancellable (file_source=0x0, cancellable=0x22a02f0) at gth-file-source.c:945
945             _g_object_unref (file_source->priv->cancellable);
(gdb) bt

+ Trace 232417

Comments:
- it works for any explicitly chosen directories using "Other..."
- it does not work with the other pre-defined directories
- it works fine with gtk+-3.6.x
- it seems to be a problem caused by changes to the gtkfilechooserbutton in gtk+-3.8.x

- reason for the issue:

dlg-find-duplicates.c:
        folder = gtk_file_chooser_get_current_folder_file (GTK_FILE_CHOOSER ( _gtk_builder_get_widget (data->builder, "location_filechooserbutton")));

returns NULL for these kind of directories

- most likely following functions don't expect folder to be NULL
- according to https://bugzilla.gnome.org/show_bug.cgi?id=701271, gtk_file_chooser_get_file should be used instead
- a quick test with the attached patch was positive, but I have not thoroughly tested whether it always select the correct directory...
- additionally the return value of that function should be checked for NULL or the following functions using that value should be aware that "folder" can be NULL