I've been having this issue for a while and I can't understand what's making it act up. I've installed the recent Optifabic launcher along with OptiFine HD U H1. I've removed some mods to test such as Appleskin and a few others and launching it without it but each time it doesn't work. I'm not sure what it could be. (I'm running Fabric 0.12.5 as well)

11:51:32.436
FabricLoader/Mixin
main
SpongePowered MIXIN Subsystem Version=0.8.4 Source=file:/C:/Users/erois/AppData/Roaming/.minecraft/libraries/net/fabricmc/sponge-mixin/0.10.6+mixin.0.8.4/sponge-mixin-0.10.6+mixin.0.8.4.jar Service=Knot/Fabric Env=CLIENT
11:51:32.513
FabricLoader/Mixin
main
Compatibility level set to JAVA_16
11:51:34.848
FabricLoader/Mixin
main
@ModifyVariable(@at("INVOKE")) Shift.BY=2 on magna.mixins.json:WorldRendererMixin::localvar$bdb000$appendBlockBreakingProgressions exceeds the maximum allowed value: 0. Increase the value of maxShiftBy to suppress this warning.
11:51:34.950
net.optifine.Log
main
[OptiFine] (Reflector) Field not found: World.tileEntitiesToBeRemoved

Are you sure there isn't any more to the logs than this? That looks to just be the the start without anything going wrong. Failing that, is there a crash report generated? If not the list of mods you're using which causes it to crash would be immensely helpful