Created a Content form mode 'example', clicked its subtab menu item and got an error

… fixed after a cache reload!

Yes the cache clearing process needs to be looked at as I have similar issues.

I get the same error but with my custom entity the cache clear does not rectify the problem.

Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException: The "resource" entity type did not specify a "fmm_edit_simple" form class. in Drupal\Core\Entity\EntityTypeManager->getFormObject() (line 212 of core/lib/Drupal/Core/Entity/EntityTypeManager.php).

The problem in #4 is that the entity type in question doesn't have an edit form class specified:

 *     "form" = {
 *       "default" = "Drupal\ascend_resource\Form\ResourceForm",
 *       "delete" = "Drupal\Core\Entity\ContentEntityDeleteForm",
 *       "revision-delete" = \Drupal\Core\Entity\Form\RevisionDeleteForm::class,
 *       "revision-revert" = \Drupal\Core\Entity\Form\RevisionRevertForm::class,
 *     },

so this code doesn't kick in:

    if ($edit_form = $entity_definition->getFormClass($route_mapper_plugin->getEditFormClass())) {

I'm not sure what the fix should be here --

a. should the route /resource/1/edit/simple request the form mode fmm_edit_*default* rather than fmm_edit_simple?

b. should the fmm_edit_simple form class get defined anyway?

Hard to know the intention when the code has to few comments!