php 7.4 Notice: Trying to access array offset on value of type null saving a contribution
Notice: Trying to access array offset on value of type null in CRM_Contribute_Form_Contribution::formRule() (line 920 of CRM\Contribute\Form\Contribution.php).
This is a new error with php 7.4 that you don't see in php 7.3 but the civi issue goes back at least to 5.24 probably farther.
Nothing special about this contribution or the config. Just a simple donation.
$this->_priceSet
is null, so the question is should it be. There's some code in submit() that seems to assume there is always at least one price set defined and I vaguely remember something somewhere about treating non-price set entries as if they were price sets, but many other places in the form check first if priceset is defined.