How to pass callback in flutter

I was not passing the callback correctly. Here is the correct syntax:

new ResultOverlay(
    isCorrect, 
    () => callAnswerPage()
)

So silly mistake :)

This is a more general answer for future viewers.

Callback types

There are a few different types of predefined callbacks:

final VoidCallback myVoidCallback = () {};
final ValueGetter<int> myValueGetter = () => 42;
final ValueSetter<int> myValueSetter = (value) {};

Notes:

VoidCallback is an anonymous function that takes no arguments and returns no value.
ValueGetter is an anonymous function that returns a value, which you provide for someone who wants to get it.
ValueSetter is an anonymous function that takes a value as an argument, which you can use to set some other value.

See this answer for more details.

When you are asked to provide a callback to an API, you can directly write the callback:

onPressed: () {},

Or you can supply the callback variable name (without parentheses):

onPressed: myVoidCallback,

It would be unnecessarily verbose to use both forms (but you could if you included the parentheses after the variable name):

onPressed: () {
  myVoidCallback();
},

This one is equivalent (but also unnecessarily verbose):

onPressed: () => myVoidCallback(),

Just use one of the "Good" forms from above.

The exception would be if you wanted to do something like call a value setter when the parameter is only asking for a void callback:

onPressed: () => myValueSetter(42),