[GTKUI] Fix unselect error with treeview selection returning None

In standalone mode treeview.get_selection returns None resulting
in an AttributeError for call to unselect_all.

deluge/ui/gtkui/torrentview.py

@@ -349,6 +349,7 @@ class TorrentView(ListView, component.Component):
     def stop(self):
         """Stops the torrentview"""
         # We need to clear the liststore
+        if self.treeview.get_selection():
             self.treeview.get_selection().unselect_all()
         self.liststore.clear()
         self.prev_status = {}