[admin] Fixed possible import error case when adding new Config

django_netjsonconfig/admin.py

@@ -122,7 +122,7 @@ def clean_templates(self):
                             config=self.data.get('config'))
         else:
             config = self.instance
-        if templates:
+        if config.backend and templates:
             Config.clean_templates(action='pre_add',
                                    instance=config,
                                    sender=config.templates,

django_netjsonconfig/tests/test_admin.py

@@ -194,3 +194,16 @@ def test_uuid_field_in_change(self):
         response = self.client.get(path)
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, 'field-id')
+
+    def test_empty_backend_import_error(self):
+        t = Template.objects.first()
+        path = reverse('admin:django_netjsonconfig_config_add')
+        params = {
+            'name': 'empty-backend',
+            'key': self.TEST_KEY,
+            'templates': str(t.pk),
+            'backend': '',
+            'config': json.dumps({'general': {'hostname': 'config'}})
+        }
+        response = self.client.post(path, params)
+        self.assertIn('errors field-backend', str(response.content))