Issue #2933125 by Tessa Bakker: Case mismatch in ExportForm.php

(cherry picked from commit 4cd7b495)

core/modules/locale/src/Form/ExportForm.php

@@ -165,7 +165,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       $header->setLanguageName($language_name);
 
       $writer = new PoStreamWriter();
-      $writer->setUri($uri);
+      $writer->setURI($uri);
       $writer->setHeader($header);
 
       $writer->open();