Dart optional vlaue as prop

Only local variables can be used like that because there are situations where the title can return null the second time it's accessed. So you could do this if you really don't want to use !

  @override
  Widget build(BuildContext context) {
    final t = title;
    return Scaffold(
      appBar: t != null
            ? AppBar(
          title: Text(t),
        )
            : null,

To give an example of why it's not guaranteed that title is not null the second time consider this program:

import 'dart:math';

main() {
  var b = B('whatever');
  b.printTitleLength();
}

class A {
  final String? title;

  A(this.title);

  void printTitleLength() {
    if (title != null) {
      print(title!.length);
    }
  }
}

class B extends A {
  B(super.title);

  @override
  String? get title => Random().nextBool() ? 'test' : null;
}

Note I had to put the ! in the print line otherwise it wouldn't compile. But also note that this possibly can throw an exception since it's possible title returns null after the null check.

use null aware operator to assign a default value when title is null, as Text widget expects a non null value.

final String? title;

appBar: title != null
        ? AppBar(
            title: Text(title??""),
          )
        : null,

You can use a switch expression to solve this now:

appbar: switch(title) { String t => AppBar(title: t), _ => null },

This works because title is copied into a new t variable, and guaranteed to be non-null. If it's null, it falls through to the catch-all.

Dart is null-safe,

so you can't define a variable without giving it an init value,

you may define an init value and it will be an empty string or if you are sure that you will give value to that variable before assigning it to the screen you can use late keyword, Note that if you misuse it and didn't assign a value before the screen is built, you will get a null error in the screen.

if you want it to accept that title could be a string or null you put the ? after String.

if you want to use that variable you need to put The null assertion operator (!)
telling dart that you are sure it won't be null during runtime

refer to this documentation to have a better understanding of Null Safety in Dart Null Safety