getByUserId(userId) {
        const entityIndex = this._userIdToEntity.get(userId);
        if (entityIndex === undefined)
            return null;
        return this.entities.get(entityIndex);
    }
Returns the entity that belongs to the player with a given user ID.
@param {number} userId - Player user ID
@returns {Player|null} Entity referenced by the user ID. `null` if no matching player.