Dragging a row between treeviews in different window groups (e.g. in different windows) was broken by commit 50aec80efbbae45f9736dc07160e7e767188d542.

On dropping a row, the following warning is now produced, and the drag and drop operation fails:

gtk_window_group_remove_window: assertion `window->group == window_group' failed

This is because gtk_window_get_group() never returns NULL; if the window doesn't have a group, the default group is returned.

Patch attached which adds a check in gtkdnd.c to see if the group is the default group. It's a tiny bit hacky, but any better fix would require API additions.