[fix] Fixed bug in preview when device has no name

The get_config method wasn't checking  if
self.name is defined before using it.

Author: nemesifier

openwisp_controller/config/base/base.py

@@ -89,7 +89,7 @@ def get_config(self):
             return config
         c = deepcopy(config)
         is_config = not any([self.__template__, self.__vpn__])
-        if 'hostname' not in c.get('general', {}) and is_config:
+        if all(('hostname' not in c.get('general', {}), is_config, self.name)):
             c.setdefault('general', {})
             c['general']['hostname'] = self.name.replace(':', '-')
         return c

openwisp_controller/config/tests/test_config.py

@@ -333,6 +333,11 @@ def test_auto_hostname(self):
         c.refresh_from_db()
         self.assertDictEqual(c.config, {'general': {}})
 
+        with self.subTest('missing name shall not raise exception'):
+            c.device.name = None
+            del c.backend_instance
+            self.assertDictEqual(c.backend_instance.config, {'general': {}})
+
     def test_config_context(self):
         config = {
             'general': {