[1.12.2; Solved] Creative tab works in workspace but not build

[edit: solved]

My mod is completed and everything works when I test via `gradlew runClient`. I'm trying to make the first build of it. After fixing multiple issues I've come across one that's left me completely stumped.

When I open the creative tabs, I crash and get the following error:
 

java.lang.AbstractMethodError: Method yuudaari/souls/common/block/BarsEndersteel.getCreativeTabToDisplayOn()Lnet/minecraft/creativetab/CreativeTabs; is abstract
    at yuudaari.souls.common.block.BarsEndersteel.getCreativeTabToDisplayOn(BarsEndersteel.java)
    at yuudaari.souls.common.CreativeTab.func_78018_a(CreativeTab.java:33)
    at net.minecraft.client.gui.inventory.GuiContainerCreative.func_147050_b(GuiContainerCreative.java:500)
    at net.minecraft.client.gui.inventory.GuiContainerCreative.func_146286_b(GuiContainerCreative.java:443)
    at net.minecraft.client.gui.GuiScreen.func_146274_d(GuiScreen.java:543)
    at net.minecraft.client.gui.inventory.GuiContainerCreative.func_146274_d(GuiContainerCreative.java:590)
    at net.minecraft.client.gui.GuiScreen.func_146269_k(GuiScreen.java:501)
    at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1757)
    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(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

All of my blocks extend the same base classes which provide their own getCreativeTabToDisplayOn function (because I'm lazy and didn't want to have to write that line on every block i make). I originally had it running setCreativeTab(CreativeTab.INSTANCE); and changed it to this in an effort to fix the problem. It didn't work, obviously.

@Override
public CreativeTabs getCreativeTabToDisplayOn() {
    return CreativeTab.INSTANCE;
}

Edited December 14, 20177 yr by Aarilight