Fix #6498: autosummary: crashed with wrong autosummary_generate setting

tests/test_ext_autosummary.py

@@ -302,3 +302,9 @@ def test_generate_autosummary_docs_property(app):
                        ".. currentmodule:: target.methods\n"
                        "\n"
                        ".. autoproperty:: Base.prop")
+
+
+@pytest.mark.sphinx('dummy', testroot='ext-autosummary',
+                    confoverrides={'autosummary_generate': ['unknown']})
+def test_invalid_autosummary_generate(app, status, warning):
+    assert 'WARNING: autosummary_generate: file not found: unknown.rst' in warning.getvalue()