The following message is added to the logs. Error: Call to a member function hasField() on bool in Drupal\commerce_donation_flow\Plugin\Commerce\CheckoutFlow\DonationCheckoutFlow->getSteps().
It appears that an item type of 'donation' is not included the order items. This causes the $donations array to be empty and consequently $donation is NULL rather than being an Order Item object. I'm unsure if this is a bug or if I'm misunderstanding how to use / configure the module.
1. Donation flow I am using the configuration for Donation Flow checkout flow which was supplied by the module.
2. I am using the Default Order type and have configured it to use the Donation Flow checkout flow (screenshot 1)
3. Configure the Donation order item type. Set the order type configured above as the order type for Donation order items. (screenshot 2)
Closed: works as designed
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
The module having a custom checkout process causes confusion. I've added a configuration page to help with this in 1.1.x. The behaviors described are referring to the standard /cart to /checkout flow. I've also added an "add donation" pane for use in this standard flow.