Once I downloaded U-Team-Core for the Useful Backpacks addon, my game would not completely load, and it would crash.

The crash takes me back to the Minecraft Launcher, as most/all crashes do, but this one gave me an error saying:

"The game crashes whilst initializing game
Error: java.land.BootstrapMethodError:
java.lang.NoClassDefFoundError:
net/minecraft/world/IEnviromentBlockReader"

Please note that minutes earlier, I was playing just fine with all sorts of mods.
Thanks, KKRadar.

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