Not super familiar with attr_json so I can't really give much insight outside of providing the error, but here's what I get:

AttrJson::Type::PolymorphicModel::TypeError in Spina::Pages#show

This AttrJson::Type::PolymorphicType can only include {Spina::Parts::Alert}, not 'Spina::Parts::Line': {"name"=>"title", "type"=>"Spina::Parts::Line", "title"=>"Title", "content"=>""}

and the line is when rendering the first item in the view.

As mentioned in the Discord, if you are experiencing this error, you can manually pin attr_json to the earlier version (gem 'attr_json', '1.5') and be good to go.