Comment 6 for bug 1666178

It's apparently a GTK bug, failing to pass a parent pointer:

MirWindowSpec* mir_create_modal_dialog_window_spec(MirConnection* connection,
                                                   int width, int height,
                                                   MirWindow* parent)
{
    mir::require(mir_window_is_valid(parent));

Although that might also be correct... Does it have a parent?