TYPO3 Core

This is the exception being thrown:

PHP Warning: Undefined property: T3\Dce\Domain\Model\Dce::$fieldsCache in /var/www/html/v11/public/typo3/sysext/extbase/Classes/DomainObject/AbstractDomainObject.php line 265

It's the _isDirty method, which causes the warning.

The easiest way to reconstruct:
1. Install DCE extension
2. Create a new DCE
3. Add

<f:debug>{dce}</f:debug>

to the main template
4. Clear system caches
5. Create a new content element on any page, based on this DCE
6. Visit the page

It is important, that debug preset (in system settings) is selected, to see the warning as exception.