This is making it almost impossible to make carts because you cannot attach any upgrades to it
Works fine if you don't but if trying to do something in creative, it takes a lot of time

Description: Unexpected error

java.lang.ClassCastException: vswe.stevescarts.blocks.tileentities.TileEntityUpgrade cannot be cast to vswe.stevescarts.blocks.tileentities.TileEntityCartAssembler
at vswe.stevescarts.blocks.BlockCartAssembler.func_180639_a(BlockCartAssembler.java:33)
at net.minecraft.client.multiplayer.PlayerControllerMP.func_187099_a(PlayerControllerMP.java:423)
at PlayerControllerOF.func_187099_a(PlayerControllerOF.java:96)
at net.minecraft.client.Minecraft.func_147121_ag(Minecraft.java:1577)
at net.minecraft.client.Minecraft.func_184117_aA(Minecraft.java:2252)
at net.minecraft.client.Minecraft.func_184118_az(Minecraft.java:2018)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1806)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1096)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:397)
at net.minecraft.client.main.Main.main(SourceFile:123)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:131)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)

What MC and mod version are you using?
What other mods are you using?

Minecraft version 1.12.2 and mod version is: StevesCarts-1.12.2-2.4.16.81

And as for other mods its: not enough items, xnet, optifine and ironchests (dependencies included)

for me 1.10.2 my game crashes when I shift-right click assembler reproduce: shift and right click on the steve's cart assembler

Can't reproduce this in 1.12.2
Can't reproduce the 1.10.2 crash either

Check if this have been fixed for you in the latest builds.

Try breaking and replacing the cart assemeber and upgrades. The incorrect tile is there.