Issue #2707689: NodeForm::actions() checks for delete access on new entities

core/lib/Drupal/Core/Form/FormBuilder.php

@@ -193,8 +193,11 @@ public function getFormId($form_arg, FormStateInterface &$form_state) {
       $form_arg = $this->classResolver->getInstanceFromDefinition($form_arg);
     }
 
-    if (!is_object($form_arg) || !($form_arg instanceof FormInterface)) {
-      throw new \InvalidArgumentException("The form argument $form_arg is not a valid form.");
+    if (!is_object($form_arg)) {
+      throw new \InvalidArgumentException(("The form class $form_arg could not be found or loaded."));
+    }
+    elseif (!($form_arg instanceof FormInterface)) {
+      throw new \InvalidArgumentException('The form argument ' . $form_arg::class . ' must be an instance of \Drupal\Core\Form\FormInterface.');
     }
 
     // Add the $form_arg as the callback object and determine the form ID.