Undefined method 'register' in 4.1.1

Hello,

I have a project I built with Middleman 3. I upgraded my installation to 4.1.1 in order to migrate from requirejs to webpack (and use the new asset pipeline extension) and now when I run middleman server, it throws the following error:

``<top (required)>’: undefined method register' for MiddlemanScavenger:Class (NoMethodError)

This happens when using middleman-scavenger and middleman-inliner.

Do you have any points or tips where I should look ? I have been googling all day about this, but sadly I came up with nothing. I will add that I am not very experimented with Ruby code.