DataMart Change Table Structure

Hello, I'm having the same issue, and it comes after saving.  The editor shows the new field just fine, but it is when trying to save that the error comes up.  It appears to load the data (all green checkmarks), then after "wrapping things up" for a long time, it pops up the error, thereby canceling the Save operation.  Any help would be appreciated, but I'm thinking this could be a Preview feature bug (along with the model layouts not saving and a couple others I've seen. I'm not bitter, just trying to send feedback so MS can get this working as intended).