fix: fallback to full user setup if we cant find the expected mount root

Signed-off-by: Robin Appelman <robin@icewind.nl>

Author: icewind1991

lib/private/Files/SetupManager.php

@@ -521,6 +521,10 @@ public function setupForPath(string $path, bool $includeChildren = false): void
 			if (is_a($mountProvider, IPartialMountProvider::class, true)) {
 				$rootId = $cachedMount->getRootId();
 				$rootMetadata = $this->fileAccess->getByFileId($rootId);
+				if (!$rootMetadata) {
+					$this->setupForUser($user);
+					return;
+				}
 				$providerArgs = new MountProviderArgs($cachedMount, $rootMetadata);
 				// mark the path as cached (without children for now...)
 				$this->setupMountProviderPaths[$mountPoint] = self::SETUP_WITHOUT_CHILDREN;