Why is StreamController not working as expected?

Question

Mobile development

Good evening.
There is a widget in which, in order for the scroll to work, commented out line 32. The scroll works, but there is no switching between buttons in the bar. I understand from the code that everything should work through the StreamController (by the way, information is transmitted through it). Of course, there is an option to implement it as described here (but I would like to figure it out).
Thanks in advance. :)