Fix Docker Server error: Not connected when deleting a node

Fix #430

gns3server/modules/docker/docker_vm.py

@@ -498,7 +498,8 @@ class DockerVM(BaseVM):
                     name=self.name, adapter_number=adapter_number))
 
         adapter.remove_nio(0)
-        yield from self._delete_ubridge_connection(adapter_number)
+        if not self._ubridge_hypervisor:
+            yield from self._delete_ubridge_connection(adapter_number)
 
         log.info(
             "Docker VM '{name}' [{id}]: {nio} removed from adapter {adapter_number}".format(