admin: avoid crash on webservice response import when single test (#89861)

tests/admin_pages/test_tests.py

@@ -1613,6 +1613,12 @@ def test_tests_webservice_response(pub):
 
     assert 'must start with http://' in resp.text
 
+    resp = app.get('/backoffice/forms/1/tests/%s/webservice-responses/' % testdef.id)
+    resp = resp.click('Import from other test')
+    resp = resp.form.submit()
+
+    assert resp.pyquery('div.error').text() == 'required field'
+
     testdef2 = TestDef.create_from_formdata(formdef, formdata)
     testdef.name = 'Second test'
     testdef2.store()

wcs/admin/tests.py

@@ -962,8 +962,8 @@ class WebserviceResponseDirectory(Directory):
         form.add(
             SingleSelectWidget,
             'testdef_id',
-            required=False,
-            options=testdef_options,
+            required=True,
+            options=[(None, '---', None)] + testdef_options,
             **{'data-autocomplete': 'true'},
         )