Ref: https://wordpress.org/support/topic/uncaught-typeerror-array_merge/#post-15862112

another fix is to check the array on line 583 :

$data_array = is_array($data) ? $data : [];
$existing_summary_array= is_array($existing_summary) ? $existing_summary : [];
$new_summary = array_merge( $existing_summary_array, $data_array );

and another fix on line 579:
$existing_summary = static::cls()->_summary ?? null;

Thanks for your PR. Please commit to dev branch

We were thinking about that fix, but got the conclusion that it is not needed.

However, it is good to have tho. Will include it.