Crash when calling insertAfter() on Silverstripe 5 in the Discount object

[juan007]

The module on version 2.1 that is compatible with silvershop/core 3.1 that at the same time has Silverstripe 5 Support is throwing a type error. This is happening because the Discount class is using the deprecated (now removed) spec for insertAfter().
This was fixed in eb070ac and tagged in 3.0.2, but given 2.1.0 supports 3.1.1 of silvershop/core it probably should be backported to the 2.1 release chain.

This is the error:
[Emergency] Uncaught TypeError: SilverStripe/Forms/FieldList::insertAfter(): Argument #2 ($item) must be of type SilverStripe/Forms/FormField, string given, called in
.../vendor/silvershop/discounts/src/Model/Discount.php on line 252