fix: ViewerEngineListener not handling UserDisconnectEvent

Author: Chubbyduck1

api/src/main/java/me/tofaa/entitylib/ve/ViewerEngineListener.java

@@ -2,6 +2,7 @@
 
 import com.github.retrooper.packetevents.event.PacketListenerAbstract;
 import com.github.retrooper.packetevents.event.PacketSendEvent;
+import com.github.retrooper.packetevents.event.UserDisconnectEvent;
 import com.github.retrooper.packetevents.protocol.packettype.PacketType;
 import com.github.retrooper.packetevents.protocol.packettype.PacketTypeCommon;
 import com.github.retrooper.packetevents.protocol.player.User;
@@ -11,6 +12,7 @@
 import me.tofaa.entitylib.utils.Check;
 import me.tofaa.entitylib.wrapper.WrapperEntity;
 
+import java.util.UUID;
 import java.util.function.Consumer;
 
 final class ViewerEngineListener extends PacketListenerAbstract {
@@ -81,6 +83,22 @@ else if (type == PacketType.Play.Server.CHUNK_DATA) {
         }
     }
 
+    @Override
+    public void onUserDisconnect(final UserDisconnectEvent event) {
+        final UUID uniqueId = event.getUser().getUUID();
+
+        // Note from Chubbyduck1: While this says it's non-null, it can actually be null in this case. Not sure why. When run without this, it would spam NPEs for me.
+        if (uniqueId == null) {
+            return;
+        }
+
+        engine.getTracked0().forEach(entity -> {
+            if (entity.hasViewer(uniqueId)) {
+                entity.removeViewer(uniqueId);
+            }
+        });
+    }
+
     private void genericSpawnCheck(int entityId, User user) {
         WrapperEntity entity = EntityLib.getApi().getEntity(entityId);
         if (entity == null) return;