fix: add null check for envelope folder node

Psalm static analysis reported that getFirstNodeById() can return
null, but getFolderForFile() was not handling this case. Added
explicit null check along with instance check to ensure type
safety and prevent potential null pointer errors.

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

lib/Service/FolderService.php

@@ -137,6 +137,9 @@ public function getFolderForFile(array $data, $identifier): Folder {
 
 		if (isset($data['settings']['envelopeFolderId'])) {
 			$envelopeFolder = $userFolder->getFirstNodeById($data['settings']['envelopeFolderId']);
+			if ($envelopeFolder === null || !$envelopeFolder instanceof Folder) {
+				throw new LibresignException($this->l10n->t('Envelope folder not found'));
+			}
 			return $envelopeFolder;
 		}