Hey, I finally managed to reproduce this bug. Seems like it only happens if you spam the End Turn button repeatedly, but that will consistently cause the game to crash. Thanks a lot for letting me know, as this was a serious bug. I will be adding it to a hotfix soon. Until then, here is how you can fix it on your own:
Add the following branch to the Player Controller:
https://i.imgur.com/4XaAJ4M.png
Delete the Queue function in BP_Action_ShowSelection. After deleting it should look like this:
https://i.imgur.com/Gln8aYv.png