only replace tracker if handle is valid

src/deluge_core.cpp

@@ -1779,18 +1779,17 @@ static PyObject *torrent_replace_trackers(PyObject *self, PyObject *args)
     return NULL;
 
   torrent_handle& h = M_torrents->at(index).handle;
-
+  if (h.is_valid()){
       std::vector<libtorrent::announce_entry> trackerlist;
-    
       std::istringstream trackers(tracker);
       std::string line;
-    
       while (std::getline(trackers, line)) {
         libtorrent::announce_entry a_entry(line);
         trackerlist.push_back(a_entry);
       }
       h.replace_trackers(trackerlist);
       h.force_reannounce();
+  }
   Py_INCREF(Py_None); return Py_None;
 }
 static PyObject *torrent_prioritize_files(PyObject *self, PyObject *args)