Steps to reproduce the issue

• create a custom field type calendar for all categories
• create an article
• enter a wrong formated date like 01.01.20261 in the custom field
• save

Expected result

i expect a joomla warning that the article can`t be saved with a wrong formated date field

Actual result

PHP Error Message
Call to a member function format() on false

System information (as much as possible)

• Joomla! 6.0.2 Stable
• PHP Version: 8.4.16-nmm1
• Database: 10.6.22-MariaDB
• Server OS: 22.04.1-Ubuntu
• Webserver: Apache

Additional comments

Screenrecord