fix: fix really dumb bug in forge

src/main/java/net/anvilcraft/ntx4core/mixin/common/ForgeFlowingFluidMixin.java

@@ -0,0 +1,35 @@
+package net.anvilcraft.ntx4core.mixin.common;
+
+import java.util.function.Supplier;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Overwrite;
+import org.spongepowered.asm.mixin.Shadow;
+
+import net.minecraft.fluid.Fluid;
+import net.minecraftforge.fluids.ForgeFlowingFluid;
+
+/**
+ * This fixes a bug in forge where `ForgeFlowingFluid.matchesType` would always assume
+ * that this.flowing != null.
+ *
+ * Fixes: https://github.com/Creators-of-Create/Create/issues/3712
+ */
+@Mixin(ForgeFlowingFluid.class)
+public class ForgeFlowingFluidMixin {
+    @Shadow(remap = false)
+    private Supplier<? extends Fluid> flowing;
+    @Shadow(remap = false)
+    private Supplier<? extends Fluid> still;
+
+    /**
+     * @reason Forge has a bullshit implementation here that NPEs when this.flowing is
+     * null.
+     * @author LordMZTE
+     */
+    @Overwrite
+    public boolean matchesType(Fluid fluid) {
+        return (still != null && still.get() == fluid)
+            || (flowing != null && flowing.get() == fluid);
+    }
+}

src/main/resources/ntx4core.mixins.json

@@ -4,7 +4,8 @@
     "compatibilityLevel": "JAVA_17",
     "minVersion": "0.8",
     "mixins": [
-        "accessor.StructureFeatureAccessor"
+        "accessor.StructureFeatureAccessor",
+        "common.ForgeFlowingFluidMixin"
     ],
     "client": [
         "client.SplashOverlayMixin",