Declaration of Drupal\flag_lists\FlagListsFlagLinkBuilder::build($entity_type_id, $entity_id, $flag_id) must be compatible with Drupal\flag\FlagLinkBuilderInterface::build($entity_type_id, $entity_id, $flag_id, $view_mode = null)

All the crashes here are related to the API update of the Flag-module. This means that the next release of the Flag Lists module will be dependent on / connected to the Flag 8.x-4.0-beta6 release.

Here is the patch I did to solve this issue in one of my projects.

Hi,

thank you for your patch!

I take this as a proof of that the patch I pushed to the -dev release is not the wrong one!
My patch also contains the extra stuff to populate the view_mode as you have figured out.

BR

/Thomas