Don’t downgrade Flutter

Problem:

This error occurs when you use a bang operator (!) on a nullable instance which wasn’t initialized.

For example:

String? string; // Nullable String

void main() {
  var len = string!.length; // Runtime error: Null check operator used on a null value
}

Solutions:

Open the logs and there must be a line pointing to a file in your project where the error occurred:

Null check operator used on a null value

#0 main (package:example/main.dart:22:16)

Once you are there, you can use any of the following ways to fix it:

• Use a local variable

  var s = string;
  if (s != null) {
    var len = s.length; // Safe 
  }
  

• Use ?. and ??

  var len = string?.length ?? 0; // Provide a default value if string was null.
  

---

The stack trace can also point to a file that doesn’t belong to your project. For example:

1. For those who are using Navigator or MediaQuery

This error also occurs when you try to access a BuildContext asynchronously.

So, you should first check if the widget is mounted before accessing BuildContext.

Future<void> foo() async {
  // Some async operation
  await compute();  

  // Check `mounted` before accessing 'context'.
  if (mounted) {
    MediaQuery.of(context).size;
    Navigator.of(context).pop();  
  }
}

---

2. For those who are using Color

You’re using

Colors.blueAccent.shade50

which doesn’t have 50th shade. If you look into the source code, you’d find:

Color get shade50 => this[50]!; // <-- This bang operator is causing the error.

To solve this error, you should use some other color which is not null, maybe the 100th shade.

Colors.blueAccent[100]
// or
Colors.blue.shade100

---

3. For those who are using FutureBuilder/StreamBuilder:

You can solve the error in two ways:

• Specify a type to your FutureBuilder/StreamBuilder

  FutureBuilder<List<int>>( // <-- type 'List<int>' is specified.
    future: _listOfInt(),
    builder: (_, snapshot) {
      if (snapshot.hasData) {
        List<int> myList = snapshot.data!; // <-- Your data
      }
      return Container();
    },
  )
  

• Use as to downcast Object to your type, say a List or Map.

  FutureBuilder(
    future: _listOfInt(),
    builder: (_, snapshot) {
      if (snapshot.hasData) {
        var myList = snapshot.data! as List<int>; // <-- Your data using 'as'
      }
      return Container();
    },
  )