Issue #3466008: Replace deprecated ::renderPlain()

src/Form/FillPdfSettingsForm.php

@@ -197,7 +197,7 @@ class FillPdfSettingsForm extends ConfigFormBase {
             '#items' => $items,
           ],
         ];
-        $this->messenger()->addError($this->renderer->renderPlain($error_message));
+        $this->messenger()->addError($this->renderer->renderInIsolation($error_message));
       }
 
       $this->logger('fillpdf')->critical('File storage %previous_scheme is no longer available.' . $count ? " $count FillPDF forms are defunct." : '', [