Fix crash when reloading a world that uses custom placed features. Closes #9979

src/main/java/net/minecraftforge/common/util/ClearableLazy.java

@@ -0,0 +1,88 @@
+package net.minecraftforge.common.util;
+
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.function.Supplier;
+
+public interface ClearableLazy<T> extends Lazy<T> {
+    void invalidate();
+
+    /**
+     * Constructs a lazy-initialized object
+     * @param supplier The supplier for the value, to be called the first time the value is needed.
+     */
+    static <T> ClearableLazy<T> of(@NotNull Supplier<T> supplier) {
+        return new ClearableLazy.Fast<T>(supplier);
+    }
+
+    /**
+     * Constructs a thread-safe lazy-initialized object
+     * @param supplier The supplier for the value, to be called the first time the value is needed.
+     */
+    static <T> ClearableLazy<T> concurrentOf(@NotNull Supplier<T> supplier) {
+        return new ClearableLazy.Concurrent<>(supplier);
+    }
+
+    /**
+     * Non-thread-safe implementation.
+     */
+    final class Fast<T> implements ClearableLazy<T> {
+        private final Supplier<T> supplier;
+        private T instance;
+
+        private Fast(Supplier<T> supplier) {
+            this.supplier = supplier;
+        }
+
+        @Nullable
+        @Override
+        public final T get() {
+            if (instance == null) {
+                instance = supplier.get();
+            }
+            return instance;
+        }
+
+        @Override
+        public void invalidate() {
+            this.instance = null;
+        }
+    }
+
+    /**
+     * Thread-safe implementation.
+     */
+    final class Concurrent<T> implements ClearableLazy<T> {
+        private final Object lock = new Object();
+        private final Supplier<T> supplier;
+        private volatile T instance;
+
+        private Concurrent(Supplier<T> supplier) {
+            this.supplier = supplier;
+        }
+
+        @Nullable
+        @Override
+        public final T get()
+        {
+            var ret = instance;
+            if (ret == null) {
+                synchronized (lock) {
+                    if (instance == null) {
+                        return instance = supplier.get();
+                    }
+                }
+            }
+            return ret;
+        }
+
+        @Override
+        public void invalidate() {
+            synchronized (lock) {
+                this.instance = null;
+            }
+        }
+    }
+}

src/main/java/net/minecraftforge/server/ServerLifecycleHooks.java

@@ -132,6 +132,10 @@ public class ServerLifecycleHooks {
         registries.registryOrThrow(Registries.BIOME).holders().forEach(biomeHolder ->
             biomeHolder.value().modifiableBiomeInfo().applyBiomeModifiers(biomeHolder, biomeModifiers)
         );
+        // Rebuild the indexed feature list
+        registries.registryOrThrow(Registries.LEVEL_STEM).forEach(levelStem -> {
+            levelStem.generator().refreshFeaturesPerStep();
+        });
         // Apply sorted structure modifiers to each structure.
         registries.registryOrThrow(Registries.STRUCTURE).holders().forEach(structureHolder ->
             structureHolder.value().modifiableStructureInfo().applyStructureModifiers(structureHolder, structureModifiers)