ContainerController has superfluous null checks for _container

ContainerController sometimes checks whether _container is null, yet most of the times it doesn't.

However, _container is only changed by the initialize() method, which assumes it's not null (through "_container.doubleClickEnabled = true"). Initialize() is only called by the constructor, which in turn is only called by the constructor of TextLineFactoryBase with a non-null Sprite.

Therefore we could safely remove the few occurrences in which _container is checked for the null value.