I had this error as well when I tested 8.0 Beta 1 and then RC on my development server.  I went into one of the skins and removed the registration of the DNNACTIONS and ACTIONBUTTONS and any lines that called it or the ACTIONBUTTONS.  The pages now load without error and the action icons still show up.

However, this means that every single legacy skin that calls this will throw and error.  I've notified 2 skin developers that I use and neither were aware of this.

I'm amazed that this is happening.  Unless there is a simple fix, everyone will have to 'fix' any skins that call this and that could be thousand of skins running on who knows how many sites.

I hope everyone has a test installation prior to applying this to their production site.  They may be very busy fixing sites.

Robert,

  We have been very open in blogs and webinars and community hangouts for months that we were removing support for many legacy components.  We removed support for Actions and ActionButtons in August of 2015, which means they weren't supported in CTP4, 5, 6 or 7, Beta 1 or the RC. 

In order to move DNN forward, we made the decision to remove many old components which are no longer needed.  We did this both to improve performance and also to trim our codebase.  The reality is that features like Actions and ActionButtons haven't played any part in DNN module action menu rendering since DNN 6 was released in 2011.  When you look at those particular controls, you find that they had dependencies on Solpart menu which hasn't been maintained or supported since 2008.