I see there's a bunch of closed issues about this...
Basically the same issue. It crashed when I had a bunch of other mods loaded, so I went and made a clean version to see if it would reproduce.
Yup. With ONLY RT (+ Forge stuff) loaded, it froze as soon as I placed a diaphanous block from the creative inventory. It didn't crash, I had to force quit.

Here's the crash report from the clean version:
crash-2018-08-22_16.50.29-client.txt

EDIT: Possibly unrelated, but the game wouldn't even launch when I was using the latest version of Forge. Could be Forge, could be Twitch/Curse, could be RT. I just stepped back to the version I was using before (.2749) and it loaded up fine. Let me know if you want that report too and I'll see if I can reproduce it.