[Bug]: Error saving calendar event after participant removal

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

After removing participant from calendar event using calendar web UI , saving event throws errors in UI and generated error messages in log like

2025-01-22 13:33:45 [...] {"reqId":"1737549225338589988","level":3,"time":"2025-01-22T12:33:45+00:00","remoteAddr":"[...]","user":"redacted","app":"PHP","method":"PUT","url":"/remote.php/dav/calendars/redacted/personal/E3F4E3A5-84F5-4189-81FB-4B712FDA36D4.ics","message":"Undefined array key \"master\" at /var/www/nextcloud/apps/dav/lib/CalDAV/TipBroker.php#104","userAgent":""[...]Firefox[...]","version":"30.0.5.1","data":{"app":"PHP"}}

2025-01-22 13:33:45 [...] {"reqId":"1737549225338589988","level":3,"time":"2025-01-22T12:33:45+00:00","remoteAddr":"[...]","user":"redacted","app":"webdav","method":"PUT","url":"/remote.php/dav/calendars/redacted/personal/E3F4E3A5-84F5-4189-81FB-4B712FDA36D4.ics","message":"{\"Exception\":\"Error\",\"Message\":\"__clone method called on non-object\",\"Code\":0,\"Trace\":[{\"file\":\"/var/www/nextcloud/3rdparty/sabre/vobject/lib/ITip/Broker.php\",\"line\":244,\"function\":\"parseEventForOrganizer\",\"class\":\"OCA\\\\DAV\\\\CalDAV\\\\TipBroker\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/CalDAV/Schedule/Plugin.php\",\"line\":616,\"function\":\"parseEvent\",\"class\":\"Sabre\\\\VObject\\\\ITip\\\\Broker\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/apps/dav/lib/CalDAV/Schedule/Plugin.php\",\"line\":204,\"function\":\"processICalendarChange\",\"class\":\"Sabre\\\\CalDAV\\\\Schedule\\\\Plugin\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/event/lib/WildcardEmitterTrait.php\",\"line\":89,\"function\":\"calendarObjectChange\",\"class\":\"OCA\\\\DAV\\\\CalDAV\\\\Schedule\\\\Plugin\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/CalDAV/Plugin.php\",\"line\":889,\"function\":\"emit\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/CalDAV/Plugin.php\",\"line\":733,\"function\":\"validateICalendar\",\"class\":\"Sabre\\\\CalDAV\\\\Plugin\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/event/lib/WildcardEmitterTrait.php\",\"line\":89,\"function\":\"beforeWriteContent\",\"class\":\"Sabre\\\\CalDAV\\\\Plugin\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":1133,\"function\":\"emit\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php\",\"line\":492,\"function\":\"updateFile\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\",\"args\":[\"*** sensitive parameters replaced ***\"]},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/event/lib/WildcardEmitterTrait.php\",\"line\":89,\"function\":\"httpPut\",\"class\":\"Sabre\\\\DAV\\\\CorePlugin\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/3rdparty/sabre/dav/lib/DAV/Server.php\",\"line\":472,\"function\":\"emit\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/apps/dav/lib/Connector/Sabre/Server.php\",\"line\":43,\"function\":\"invokeMethod\",\"class\":\"Sabre\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/apps/dav/lib/Server.php\",\"line\":371,\"function\":\"start\",\"class\":\"OCA\\\\DAV\\\\Connector\\\\Sabre\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/apps/dav/appinfo/v2/remote.php\",\"line\":19,\"function\":\"exec\",\"class\":\"OCA\\\\DAV\\\\Server\",\"type\":\"->\"},{\"file\":\"/var/www/nextcloud/remote.php\",\"line\":146,\"args\":[\"/var/www/nextcloud/apps/dav/appinfo/v2/remote.php\"],\"function\":\"require_once\"}],\"File\":\"/var/www/nextcloud/apps/dav/lib/CalDAV/TipBroker.php\",\"Line\":104,\"message\":\"__clone method called on non-object\",\"exception\":{},\"CustomMessage\":\"__clone method called on non-object\"}","userAgent":"[...]Firefox[...]","version":"30.0.5.1"}

Event is not updated in db (participant is not removed after page reload).

NC server and calendar app build from sources:

• server: stable30 (ad96892cce103b012765d62e12e8554bebd5a4f5)
• 3rdparty: v30.0.5
• calendar: stable5.0 (693b2db9a6e342d7ca40032d17d99658d9d37a73)

Same problem in v30.0.4. No such problem in v29.0.2.

Steps to reproduce

1. In NC 30 web UI create event and add 1 participant other than you. Save an event.
2. In NC 30 web UI open created event (detailed view) and remove other participant and press button to update event. Alternative: sync event to Thunderbird, remove other participant from event and sync calendar.
3. See errors in log.

Expected behavior

Event should be saved without errors and no other participant should be present in saved event.

Nextcloud Server version

30

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.2

Web server

Apache (supported)

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

Start in apps/dav/lib/CalDAV/TipBroker.php at line 104, then follow the stack trace through sabre/vobject's ITip Broker and CalDAV scheduling plugin. Reproduce the participant-removal update in Nextcloud 30 and verify that the event saves without errors and the participant remains removed after reload.