A recent update to Mendix has resulted in the mentioned errors no longer being thrown. The buttons that have been the cause of the errors in the past have had their action property set from ‘Create Object’ to ‘Nothing’ in the template. We are in the process of updating the content of the course to reflect this change.
With regards to not being able to see the preview, that one is a bit more tricky as you are not seeing any errors. It is therefore difficult to ascertain what is going on. I’m afraid I don’t have a solution for you with this amount of information.
At this point, I would advise you to start a new project and use the Academy Blank App as a starting point.