fix(files_sharing): don't crash CleanupShareTarget when mount info is missing

The repair step "Cleanup share names with false conflicts" fatals with
"Call to a member function getRootId() on null" when a problem share's
old target has no entry in the recipient's cached mounts (e.g. the
recipient has not logged in since the share was created).

Guard the lookup the same way the generateUniqueTarget callback already
does and skip the mount cache refresh in that case - the share row is
already updated by moveShare() and the cache is rebuilt on the user's
next login.

Fixes #63494

Signed-off-by: Baki Burak Öğün <63836730+bakiburakogun@users.noreply.github.com>

Author: bakiburakogun

apps/files_sharing/lib/Repair/CleanupShareTarget.php

@@ -118,16 +118,18 @@ public function run(IOutput $output) {
 					$oldMountPoint = "/{$recipient->getUID()}/files$oldTarget/";
 					$newMountPoint = "/{$recipient->getUID()}/files$newTarget/";
 
-					/** @var ICachedMountInfo $mount */
-					$mount = $userMounts[$oldMountPoint];
-					$userMounts[$newMountPoint] = $mount;
-					unset($userMounts[$oldMountPoint]);
-
-					$this->userMountCache->removeMount($oldMountPoint);
-					$this->userMountCache->addMount($recipient, $newMountPoint, new CacheEntry([
-						'fileid' => $mount->getRootId(),
-						'storage' => $mount->getStorageId(),
-					]), $mount->getMountProvider(), $mount->getMountId());
+					/** @var ICachedMountInfo|null $mount */
+					$mount = $userMounts[$oldMountPoint] ?? null;
+					if ($mount !== null) {
+						$userMounts[$newMountPoint] = $mount;
+						unset($userMounts[$oldMountPoint]);
+
+						$this->userMountCache->removeMount($oldMountPoint);
+						$this->userMountCache->addMount($recipient, $newMountPoint, new CacheEntry([
+							'fileid' => $mount->getRootId(),
+							'storage' => $mount->getStorageId(),
+						]), $mount->getMountProvider(), $mount->getMountId());
+					}
 				}
 			} catch (\Exception $e) {
 				$msg = 'error cleaning up share target: ' . $e->getMessage();