Merge pull request #54259 from nextcloud/backport/54240/stable31

[stable31] fix(caldav): get UID value

Author: nickvergessen

apps/dav/lib/CalDAV/Schedule/IMipService.php

@@ -1110,7 +1110,7 @@ public function createInvitationToken(Message $iTipMessage, VEvent $vevent, int
 		$sequence = $iTipMessage->sequence;
 		$recurrenceId = isset($vevent->{'RECURRENCE-ID'}) ?
 			$vevent->{'RECURRENCE-ID'}->serialize() : null;
-		$uid = $vevent->{'UID'};
+		$uid = $vevent->{'UID'}?->getValue();
 
 		$query = $this->db->getQueryBuilder();
 		$query->insert('calendar_invitations')