Document live DM device panels and release their UI callback

tools/chatgui/src/mainwindow.cpp

@@ -238,6 +238,7 @@ MainWindow::~MainWindow() {
     gui_bridge_set_dm_start_result_cb(nullptr);
     gui_bridge_set_dm_msg_received_cb(nullptr);
     gui_bridge_set_dm_messages_cb(nullptr);
+    gui_bridge_set_dm_members_cb(nullptr);
     gui_bridge_set_dm_delete_result_cb(nullptr);
     gui_bridge_set_msg_received_cb(nullptr);
     gui_bridge_set_channel_updated_cb(nullptr);