json cell: always reset json context variable (#16463)

combo/data/models.py

@@ -829,6 +829,7 @@ class JsonCellBase(CellBase):
                 if varname in context['request'].GET and varname not in context:
                     context[varname] = context['request'].GET[varname]
         self._json_content = None
+        extra_context['json'] = None
         try:
             url = utils.get_templated_url(self.url, context)
         except utils.UnknownTemplateVariableError:

tests/test_cells.py

@@ -200,7 +200,7 @@ def test_json_cell():
     with mock.patch('combo.utils.requests.get') as requests_get:
         context = cell.get_cell_extra_context(Context({}))
         # can't get URL, 'foobar' variable is missing
-        assert 'json' not in context
+        assert context['json'] == None
         assert requests_get.call_count == 0
     request = RequestFactory().get('/')
     request.user = User(username='foo', email='foo@example.net')