Add CREATE_FOG BossFlag

api/src/main/java/com/viaversion/viaversion/api/data/entity/EntityTracker.java

@@ -73,6 +73,7 @@ public interface EntityTracker {
      *
      * @param id entity id
      * @return stored entity data if an entity with the id is tracked, else null
+     * @throws IllegalArgumentException if entitiy data storage has not been enabled via the implementation
      */
     @Nullable StoredEntityData entityData(int id);
 
@@ -81,6 +82,7 @@ public interface EntityTracker {
      *
      * @param id entity id
      * @return stored entity data if it has previously been initialized by {@link #entityData(int)}
+     * @throws IllegalArgumentException if entitiy data storage has not been enabled via the implementation
      */
     @Nullable StoredEntityData entityDataIfPresent(int id);
 

api/src/main/java/com/viaversion/viaversion/api/legacy/bossbar/BossFlag.java

@@ -25,7 +25,8 @@ package com.viaversion.viaversion.api.legacy.bossbar;
 public enum BossFlag {
 
     DARKEN_SKY(1),
-    PLAY_BOSS_MUSIC(2);
+    PLAY_BOSS_MUSIC(2),
+    CREATE_FOG(4);
 
     private final int id;
 

common/src/main/java/com/viaversion/viaversion/data/entity/EntityTrackerBase.java

@@ -70,12 +70,14 @@ public class EntityTrackerBase implements EntityTracker, ClientEntityIdChangeLis
 
     @Override
     public @Nullable StoredEntityData entityData(int id) {
+        Preconditions.checkArgument(entityData != null, "Entity data storage has to be explicitly enabled via the constructor");
         EntityType type = entityType(id);
         return type != null ? entityData.computeIfAbsent(id, s -> new StoredEntityImpl(type)) : null;
     }
 
     @Override
     public @Nullable StoredEntityData entityDataIfPresent(int id) {
+        Preconditions.checkArgument(entityData != null, "Entity data storage has to be explicitly enabled via the constructor");
         return entityData.get(id);
     }