Use reflection to unfreeze the built-in item registry to prevent star…

…tup error.

src/main/java/net/slqmy/firework_wars_plugin/items/manager/CustomItemManager.java

@@ -1,26 +1,41 @@
 package net.slqmy.firework_wars_plugin.items.manager;
 
+import net.minecraft.core.DefaultedMappedRegistry;
+import net.minecraft.core.MappedRegistry;
+import net.minecraft.core.registries.BuiltInRegistries;
 import net.minecraft.world.item.Item;
 import net.minecraft.world.item.Items;
 import net.slqmy.firework_wars_plugin.FireworkWarsPlugin;
 import net.slqmy.firework_wars_plugin.items.FireworkRifleItem;
 import net.slqmy.firework_wars_plugin.items.RifleAmmo;
 import net.slqmy.firework_wars_plugin.items.nms.CustomCrossbow;
 
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.lang.reflect.Proxy;
 import java.util.HashMap;
+import java.util.IdentityHashMap;
 import java.util.Map;
 
 public class CustomItemManager {
+  private final FireworkWarsPlugin plugin;
+
   private final Map<String, AbstractItem> itemRegistry = new HashMap<>();
   private final Map<String, Item> nmsItemRegistry = new HashMap<>();
 
   public CustomItemManager(FireworkWarsPlugin plugin) {
+    this.plugin = plugin;
+
     registerItem(new FireworkRifleItem(plugin));
     registerItem(new RifleAmmo(plugin));
 
+    reopenItemRegistry();
+
     registerNMSItem(
         "firework_rifle_crossbow",
         new CustomCrossbow(CustomCrossbow.PROPERTIES, getItem("firework_rifle_ammo")));
+
+    BuiltInRegistries.ITEM.freeze();
   }
 
   public Map<String, AbstractItem> getItemRegistry() {
@@ -47,4 +62,19 @@ private void registerNMSItem(String id, Item item) {
     Items.registerItem(id, item);
     nmsItemRegistry.put(id, item);
   }
+
+  private void reopenItemRegistry() {
+    try {
+      Field frozenField = MappedRegistry.class.getDeclaredField("frozen");
+      Field unregisteredHoldersField = MappedRegistry.class.getDeclaredField("unregisteredIntrusiveHolders");
+
+      frozenField.setAccessible(true);
+      unregisteredHoldersField.setAccessible(true);
+
+      frozenField.setBoolean(BuiltInRegistries.ITEM, false);
+      unregisteredHoldersField.set(BuiltInRegistries.ITEM, new IdentityHashMap<>());
+    } catch (IllegalAccessException | NoSuchFieldException e) {
+      plugin.getLogger().severe("Failed to reopen the item registry! " + e.getMessage());
+    }
+  }
 }