[11.x] Change closure to arrow function in Dispatcher

[azibom]

Hi
I changed closure to arrow function in Dispatcher to make the code more readable.

[MrPunyapal]

Arrow functions must return something 👀
It's not the return type friendly.
And we need a future of types 🤞

[rodrigopedra]

to make the code more readable

It is perfectly readable in its current state.

Furthermore, before the callback returned null, now it returns mixed.

[driesvints]

This code unfortunately isn't the same as @rodrigopedra indicated.