bool True, if a singleton of the specified type exists in the current World.
Checks whether a singleton component of the specified type exists.
The SystemAPI singleton methods build their query with the default EntityQueryOptions
flags, which exclude entities that have the Disabled component and entities that have the
Prefab component. So if the only entity that has component T is disabled or is
an entity prefab, this method returns false, and SystemAPI.GetSingleton throws an exception.
To find a singleton component on a disabled entity or on an entity prefab, create an EntityQuery with
the EntityQueryOptions.IncludeDisabledEntities or EntityQueryOptions.IncludePrefab option,
then call EntityQuery.HasSingleton on that query.
Not working in Entities.ForEach, IJobEntity, and Utility methods