Yeah, also there's now a behavior difference between the bluetooth applet and the control center.  Previously, the plugin's widget lived for the duration of the bluetooth applet's dialog.  But the control center destroys and recreates the plugin widgets every time the bluetooth device's properties change, which is something that the NM plugin does when it calls 'Connect' for DUN devices to inspect the modem and find out whether it's GSM or CDMA.

Since this is a long-running operation, and the NM plugin's widget gets destroyed halfway through, things don't end well.  That's commit 844dd9e3 in gnome-control-center, see cc_bluetooth_panel_update_properties() where it calls remove_extra_setup_widgets().  Which does:

static void
remove_extra_setup_widgets (CcBluetoothPanel *self)
{
	GtkWidget *box;

	box = WID ("additional_setup_box");
	gtk_container_forall (GTK_CONTAINER (box), (GtkCallback) gtk_widget_destroy, NULL);
	gtk_widget_hide (WID ("additional_setup_box"));
}

and boom, our NM plugin widget is gone, and the long-running operations start using already freed data when they complete.

*** This bug has been marked as a duplicate of bug 678018 ***