错误处理伪已经存在flutter firebase

Question

我有一个关于 try/catch with dart 的小问题。

如何添加脚本以检查我的函数 SignUp 中是否已存在伪代码？

  _signUp(String _email, String _password, String _pseudo) async {
    try {
      //Create Get Firebase Auth User
      if (_formKey.currentState.validate()) {
        await auth.createUserWithEmailAndPassword(
            email: _email, password: _password);

        //problem here 
        users.get().then((querySnapshot) {
          querySnapshot.docs.forEach((result) {
            var pseudoDoc = result.data()['pseudo'];
            if (pseudoDoc != _pseudo) {
              users.doc(auth.currentUser.uid).set({
                'pseudo': _pseudo,
                'email': _email,
              });
            } else {
              throw 'Pseudo already exist';
            }
          });
        });
        //
      }
    } on FirebaseAuthException catch (error) {
      CoolAlert.show(
        context: context,
        type: CoolAlertType.error,
        text: error.message,
      );
    }
  }
}```