I'm sure you guys already know about this one, but it's not posted anywhere, so I thought maybe I should post it just in case.

24.11 18:33:26 [Server] ERROR Caught exception from lightsabers 24.11 18:33:26 [Server] INFO java.lang.NoClassDefFoundError: net/minecraft/client/gui/GuiScreen

Full report:
crash-2019-11-24_18.33.26-server.txt

I too am having this issue, simply making a server with the lightsaber mod causes this crash

Fixed in 1.2.2