diff -u b/core/modules/text/src/Plugin/Field/FieldType/TextFieldItemList.php b/core/modules/text/src/Plugin/Field/FieldType/TextFieldItemList.php
--- b/core/modules/text/src/Plugin/Field/FieldType/TextFieldItemList.php
+++ b/core/modules/text/src/Plugin/Field/FieldType/TextFieldItemList.php
@@ -16,7 +16,7 @@
   public function defaultValuesFormValidate(array $element, array &$form, FormStateInterface $form_state) {
     if ($allowed_formats = $this->getSetting('allowed_formats')) {
       if (!empty($allowed_formats)) {
-        $field_name = $this->fieldDefinition->getName();
+        $field_name = $this->definition->getName();
         $submitted_values = $form_state->getValue(['default_value_input', $field_name]);
         foreach ($submitted_values as $delta => $value) {
           if (!in_array($value['format'], $allowed_formats)) {