network: don't try to update device if it doesn't exist

All of the following code assumes that self->device is valid, so we need
to skip over it. It's confusing, but this is a multipurpose dialog and
self->device is optional when creating the dialog. E.g. when modifying
VPN configuration, we update just the configuration, not an NMDevice.

panels/network/connection-editor/net-connection-editor.c

@@ -271,6 +271,11 @@ apply_clicked_cb (NetConnectionEditor *self)
 
         eap_method_ca_cert_ignore_save (self->connection);
 
+        if (!self->device) {
+                update_complete (self, TRUE);
+                return;
+        }
+
         if (self->is_new_connection) {
                 nm_client_add_connection_async (self->client,
                                                 self->orig_connection,