Issue #3130107: Extend unit test coverage for LanguageNegotiationContentEntity

core/lib/Drupal/Core/TypedData/DataReferenceBase.php

@@ -54,6 +54,9 @@ public function setValue($value, $notify = TRUE) {
    * {@inheritdoc}
    */
   public function getString() {
+    if (!method_exists($this, 'getType')) {
+      throw new \BadMethodCallException(get_class($this) . '::getType() not implemented');
+    }
     return (string) $this->getType() . ':' . $this->getTargetIdentifier();
   }