correction warning not set structure

api/DOIModifier.php

@@ -150,7 +150,7 @@ function changeDoi(string $halId_s, string $newDOI, string $portail='', bool $de
         'sendTei' => $originalFile->getValue(),
         'originalTEI' => $originalTEI,
         'newTEI' => $debug?$originalFile->getValue():$newTEI,
-        'structure' => $_SESSION['structure'],
+        'structure' => $_SESSION['structure'] ?? '',
         'debug' => $debug,
         'errorMessage' => $errorMessage,
     ];