Handle possible null value for file in OC\Files\SimpleFS\NewSimpleFile

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/private/Files/SimpleFS/NewSimpleFile.php

@@ -136,6 +136,10 @@ class NewSimpleFile implements ISimpleFile {
 	 * @throws NotFoundException
 	 */
 	private function checkFile(): void {
+		if (!$this->file) {
+			throw new NotFoundException('File not set');
+		}
+
 		$cur = $this->file;
 
 		while ($cur->stat() === false) {