How to convert these block of codes into non-nullable statement/s in dart?

Question

I am trying to convert these lines of code into non-nullable statements, but encountering the error...

The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type. Try adding either a return or a throw statement at the end.

             child: BlocBuilder<LoginBloc, LoginState>(
                builder: (context, state) {
                  if (state is LoginInitialState) {
                    return buildInitialUi();
                  } else if (state is LoginLoadingState) {
                    return buildLoadingUi();
                  } else if (state is LoginFailState) {
                    return buildFailureUi(state.message);
                  } else if (state is LoginSuccessState) {
                    emailCntrlr.text = "";
                    passCntrlr.text = "";
                    return Container();
                  }
                },
              ),

Solution 1

Try this

child: BlocBuilder<LoginBloc, LoginState>(
                builder: (context, state) {
                  if (state is LoginInitialState) {
                    return buildInitialUi();
                  } else if (state is LoginLoadingState) {
                    return buildLoadingUi();
                  } else if (state is LoginFailState) {
                    return buildFailureUi(state.message);
                  } else if (state is LoginSuccessState) {
                    emailCntrlr.text = "";
                    passCntrlr.text = "";
                    return Container();
                  }
                  return Container(); // In case none of the if statements is true
                },
              ),

Solution 2

Try This

After String add "?" and return null; Like This

class ValidationMixin {
  String? validateEmail(String value) {
    if (!value.contains('@')) {
      return 'Please enter a Valid Email';
    }
    return null;
  }
}

Comments

[Julia]

Couldn't you do what the error message says and add a throw statement at the end?

[Nikash Deka]

It worked, thanks. However, while accessing the property of the state (i.e., state.message), an error occurred... "The argument type 'String?' can't be assigned to the parameter type 'String'." After the null-safety upgrade in the SDK I am finding such errors in many places!

[YoBo]

you can set default value if the message is nullable: state.message ?? ''

[Nikash Deka]

How to use packages that are not upgraded to null-safety?