Fixes rare issue when adding a link. Fixes #573.

Author: grossmj

gns3/link.py

@@ -257,6 +257,10 @@ def newNIOSlot(self, node_id, port_id):
         :param port_id: port identifier
         """
 
+        # in very rare cases link is already deleted
+        if self is None:
+            return
+
         # check that the node is connected to this link as a source
         if node_id == self._source_node.id() and port_id == self._source_port.id():
             self._source_nio_active = True