TYPO3 Core

../typo3/sysext/backend/Classes/View/ValueFormatter/FlexFormValueFormatter.php
Line 56
$processedSheets = $this->getProcessedSheets($flexFormDataStructure, $flexFormDataArray['data']);

Throws an error if a new created field is a flexform field.

My solution:
$processedSheets = $this->getProcessedSheets($flexFormDataStructure, $flexFormDataArray['data'] ?? []);

Steps to reproduce:
Add to an existing TYPO3 Installation a new flexform field, e.g. tt_content table.
Then view history.