I tried running 17w50a for the first time today and when creating a new single player world the mincraft window just crash after "preparing the spawn area"
The same world works perfectly fine in 1.12.2.
Even when creating an other world it won't work.
Then I tried making an world in 1.12.2 and opening it in 17w50a that didn't work either.
When you watch the video and see the crash report from mac os it is the pastebin link.
https://pastebin.com/hpFx28hg
Unfortunately i will be going away for about 2 weeks and wont have access to internet that's why I probably wont respond that quickly.