--- a/core/lib/Drupal/Core/Field/PluginSettingsBase.php
+++ b/core/lib/Drupal/Core/Field/PluginSettingsBase.php
@@ -99,7 +99,7 @@
    */
   public function getThirdPartySettings($module = NULL) {
     if ($module) {
-      return isset($this->thirdPartySettings[$module]) ? $this->thirdPartySettings[$module] : NULL;
+      return isset($this->thirdPartySettings[$module]) ? $this->thirdPartySettings[$module] : [];
     }
     return $this->thirdPartySettings;
   }