fix: only use file when exists

Signed-off-by: Vitor Mattos <[email protected]>

Author: vitormattos

lib/Service/FileService.php

@@ -299,8 +299,7 @@ public function isLibresignFile(int $nodeId): bool {
 
 	private function loadFileMetadata(): void {
 		if (
-			!$this->file
-			|| $this->file->getNodeType() !== 'file'
+			($this->file instanceof File && $this->file->getNodeType() !== 'file')
 			|| !$content = $this->getFileContent()
 		) {
 			return;