Properly remove manual viewers

Signed-off-by: TheMode <themode@outlook.fr>

src/main/java/net/minestom/server/entity/Entity.java

@@ -878,6 +878,7 @@ public class Entity implements Viewable, Tickable, TagHandler, PermissionHandler
     private void removeFromInstance(Instance instance) {
         EventDispatcher.call(new RemoveEntityFromInstanceEvent(instance, this));
         instance.getEntityTracker().unregister(this, position, trackingTarget, trackingUpdate);
+        this.viewEngine.forManuals(this::removeViewer);
     }
 
     /**

src/main/java/net/minestom/server/utils/ViewEngine.java

@@ -77,6 +77,12 @@ public final class ViewEngine {
         }
     }
 
+    public void forManuals(@NotNull Consumer<Player> consumer) {
+        synchronized (mutex) {
+            this.manualViewers.forEach(consumer);
+        }
+    }
+
     public boolean hasPredictableViewers() {
         // Verify if this entity's viewers can be predicted from surrounding entities
         synchronized (mutex) {