In certain circumstances the $items variable in uc_order_line_item_add() may not be an array, so with PHP 7.2 the count($items) check will throw an error.
Closed: works as designed
3.0
Code
The issue particularly affects sites running on PHP version 7.2.0 or later.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Well as I explained in #6, the error is not in Ubercart, but is in some other module that improperly modifies $items inside of a hook_uc_line_item_data_alter() call.
Because no further information has been given about how to reproduce the error or which other module (contrib or custom) is being used, I can't help with this.