I just updated to Mojave (10.14) and RW6 does not want to go in internal preview : it makes a loop and displays “exporting”, then loop again and again. The only solution for the moment is to preview in your browser.
Expecting that the update will be soon there to fix it, because it’s not really practical !
Unfortunately, RW 6 was not supported starting with High Sierra
You’re not going to get updates/support for RW6.
Try previewing in a browser. This should still allow you to use RW 6, just can not preview it in the app.