The GSList head changes when buttons are added to the group, so it
should not be used that way. This avoids a segfault when closing the
prefs dialog:

Program terminated with signal SIGSEGV, Segmentation fault.
#0  gtk_radio_button_destroy (widget=0x5a4874b6d730) at ../gtk/gtk/gtkradiobutton.c:609
609	      tmp_button->priv->group = priv->group;