Issue #1983164 by dawehner, larowlan: Entity Forms in ajax requests don't find the route.

core/lib/Drupal/Core/Controller/AjaxController.php

core/lib/Drupal/Core/Controller/HtmlFormController.php

core/lib/Drupal/Core/HttpKernel.php

core/lib/Drupal/Core/Routing/Enhancer/AjaxEnhancer.php

...odules/views_ui/lib/Drupal/views_ui/Tests/PreviewTest.php

...iews_ui/lib/Drupal/views_ui/ViewPreviewFormController.php

core/modules/views_ui/lib/Drupal/views_ui/ViewUI.php

core/modules/views_ui/views_ui.theme.inc

core/tests/Drupal/Tests/Core/Controller/TestController.php

core/tests/Drupal/Tests/Core/HttpKernelTest.php