fix the tests by falling back to a empty dict on AttributeError

sphinx/ext/napoleon/docstring.py

@@ -1131,7 +1131,11 @@ class NumpyDocstring(GoogleDocstring):
         func_name1, func_name2, :meth:`func_name`, func_name3
 
         """
-        inventory = getattr(self._app.builder.env, "intersphinx_inventory", {})
+        try:
+            inventory = self._app.builder.env.intersphinx_inventory
+        except AttributeError:
+            inventory = {}
+
         items = []
 
         def parse_item_name(text: str) -> Tuple[str, str]:

tests/test_ext_napoleon_docstring.py

@@ -1365,6 +1365,7 @@ otherfunc : relationship
 
         config = Config()
         app = mock.Mock()
+        app.builder.env = None
         actual = str(NumpyDocstring(docstring, config, app, "method"))
 
         expected = """\
@@ -1379,6 +1380,27 @@ numpy.multivariate_normal(mean, cov, shape=None, spam=None)
 """
         self.assertEqual(expected, actual)
 
+        config = Config()
+        app = mock.Mock()
+        app.builder.env.intersphinx_inventory = {
+            "py:func": {"funcs": (), "otherfunc": ()},
+            "py:meth": {"some": (), "other": ()},
+        }
+        actual = str(NumpyDocstring(docstring, config, app, "method"))
+
+        expected = """\
+numpy.multivariate_normal(mean, cov, shape=None, spam=None)
+
+.. seealso::
+
+   :meth:`some`, :meth:`other`, :func:`funcs`
+   \n\
+   :func:`otherfunc`
+       relationship
+"""
+        self.assertEqual(expected, actual)
+
+
     def test_colon_in_return_type(self):
         docstring = """
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