Fix flat level generator breaking after first usage

src/mixins/java/org/spongepowered/common/mixin/core/world/level/levelgen/flat/FlatLevelGeneratorSettingsMixin.java

@@ -0,0 +1,50 @@
+/*
+ * This file is part of Sponge, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) SpongePowered <https://www.spongepowered.org>
+ * Copyright (c) contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package org.spongepowered.common.mixin.core.world.level.levelgen.flat;
+
+import net.minecraft.world.level.levelgen.flat.FlatLevelGeneratorSettings;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Redirect;
+import org.spongepowered.asm.mixin.injection.Slice;
+
+import java.util.List;
+
+@Mixin(FlatLevelGeneratorSettings.class)
+public abstract class FlatLevelGeneratorSettingsMixin {
+
+    @Redirect(method = "adjustGenerationSettings", at = @At(value = "INVOKE", target = "Ljava/util/List;set(ILjava/lang/Object;)Ljava/lang/Object;"),
+            slice = @Slice(
+                    from = @At(value = "FIELD", target = "Lnet/minecraft/world/level/levelgen/Heightmap$Types;MOTION_BLOCKING:Lnet/minecraft/world/level/levelgen/Heightmap$Types;"),
+                    to = @At(value = "FIELD", target = "Lnet/minecraft/world/level/levelgen/GenerationStep$Decoration;TOP_LAYER_MODIFICATION:Lnet/minecraft/world/level/levelgen/GenerationStep$Decoration;")
+            )
+    )
+    private <E> E impl$preventLayerOverwrite(final List<?> instance, final int index, final E element) {
+        //Not really sure what vanilla is trying to do here but this doesn't make much sense and,
+        //we hit NPE on the next call to this method. This doesn't happen on vanilla as it always
+        //creates a new instance.
+        return null;
+    }
+}

src/mixins/resources/mixins.sponge.core.json

@@ -217,6 +217,7 @@
         "world.level.levelgen.NoiseGeneratorSettingsMixin",
         "world.level.levelgen.WorldOptionsMixin",
         "world.level.levelgen.flat.FlatLayerInfoMixin",
+        "world.level.levelgen.flat.FlatLevelGeneratorSettingsMixin",
         "world.level.levelgen.structure.LegacyStructureDataHandlerMixin",
         "world.level.portal.PortalShapeMixin",
         "world.level.saveddata.MapIdTrackerMixin",