Fatal error: Call to a member function extractFormValues() : Drupal 8 beta 14
This question is old but I stumbled across this issue as well, so posting the solution for future googlers..

Basically, you need to init the form_state within the buildForm method with:

if (!$form_state-&gt;has('entity_form_initialized')) {
  $this-&gt;init($form_state);
}


I found this solution here: https://withpulp.com/blog/drupal-forms-api/#initialize-the-form-state

When extending the ContentEntityForm class the parent buildForm normally handles this for you, but if for some reason you choose not to extend the buildForm you will need to init the form_state yourself.