ErrorException Trying to access array offset on value of type null

The PHP Version is indeed the problem. On the previous host it was PHP 7.3 but the new one run 7.4 so it caused the problem.

Now I have downgraded to PHP 7.3.1 and the application now works fine.

Thanks so much.

Your problem was caused by a change to the php new version. If you do not know which page caused this error, you can use the Symfony test. Maybe this link will help you: