Null safety in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the null safety error

Which option will cause a compile-time error due to null safety in Flutter?

Flutter

String name = null;
print(name.length);

A String? name = null; print(name?.length);

B String name = null; print(name.length);

C String name = 'Flutter'; print(name.length);

D String? name = 'Dart'; print(name!.length);

💡 Hint

Check if the variable type allows null values.

Option B declares a non-nullable String but assigns null, which is not allowed in null safety.

❓ ui_behavior

intermediate

Null safety impact on UI rendering

What will be displayed when this Flutter widget runs?

Flutter

String? message;

Widget build(BuildContext context) {
  return Text(message ?? 'No message');
}

A Displays 'No message'

B Displays nothing (empty space)

C Throws a runtime error

D Displays 'null'

💡 Hint

Look at the null-coalescing operator usage.

The '??' operator provides a fallback string when 'message' is null, so 'No message' is shown.

❓ lifecycle

advanced

Null safety and late initialization

What happens if you access a 'late' variable before it is initialized?

Flutter

late String title;

void printTitle() {
  print(title);
}

A Throws LateInitializationError

B Prints null

C Prints empty string

D Prints 'title'

💡 Hint

Consider what 'late' means in Dart null safety.

Accessing a 'late' variable before initialization causes a runtime LateInitializationError.

❓ navigation

advanced

Null safety in route arguments

Given this code, what will happen if 'args' is null?

Flutter

final args = ModalRoute.of(context)?.settings.arguments as String;

print(args.length);

A Prints 0

B Throws a NoSuchMethodError

C Throws a TypeError

D Prints null

💡 Hint

Check the cast and null safety behavior.

Casting null to String with 'as String' causes a TypeError at runtime.

🔧 Debug

expert

Debugging null safety error in widget state

Why does this Flutter stateful widget throw a null safety error?

Flutter

class MyWidget extends StatefulWidget {
  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  late String title;

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Text(title);
  }
}

A Because 'title' should be nullable String

B Because 'initState' is missing super.initState() call

C Because Text widget cannot display variables

D Because 'title' is late but never initialized before use

💡 Hint

Check when 'title' is assigned a value.

The 'late' variable 'title' is used in build without initialization, causing LateInitializationError.