association_scope is private in Rails 6 (since https://github.com/rails/rails/commit/0b4cfa2ba3e978386240e0e56a409616cc32fd02):

[...]
NoMethodError: private method `association_scope' called for #<ActiveRecord::Associations::HasManyAssociation:0x00000000100e57f8>
    lib/core_extensions.rb:41:in `block in before_destroy'
    lib/core_extensions.rb:40:in `each'
    lib/core_extensions.rb:40:in `before_destroy'
[...]