1. In Preferences -> Network, specify some low "Max KB/S", so the transfer won't finish too soon,
2. Connect to a remote host (tcpsvd -vE 0.0.0.0 21 ftpd -w /tmp will do),
3. Start uploading a file,
4. Double-click on any remote directory, while the transfer is already and still in progress.

listbox.c: ERROR, could not retrieve filename(s)...
Segmentation fault

Instead of 4, you can also right-click->Refresh on the remote pane - all items will disappear with "Not connected*" message above the pane.
At this point you can reconnect and everything works fine, until the next transfer.

Also, the segfault doesn't happen with local transfers ("file:///" as "Host").

I found only one bug report about it: https://bugs.launchpad.net/ubuntu/+source/gftp/+bug/1461428