Issue #2940301 by SpadXIII, timplunkett: Warning: Illegal offset type in isset or empty

src/Plugin/Block/FieldBlock.php

@@ -201,7 +201,7 @@ class FieldBlock extends BlockBase implements ContainerFactoryPluginInterface {
       }
       else {
         if (empty($formatter_id)) {
-          $formatter_id = reset($formatter_options);
+          $formatter_id = key($formatter_options);
         }
         $form['formatter']['id'] = [
           '#title' => $this->t('Formatter'),