Why I'm having an error with my UserModel to register?

Question

I'm trying to save a photo profile for the user of my application with a long process with id and everything, I followed a tutorial but I'm having errors. Thank you for helping me !

So this is my code in my profile edit screen:

class ProfileEditScreen extends StatefulWidget {

  @override
  _ProfileEditScreenState createState() => _ProfileEditScreenState();
}

class _ProfileEditScreenState extends State<ProfileEditScreen> {
  UserModel? currentUser;

  @override
  Widget build(BuildContext context) {
    Size size = MediaQuery
        .of(context)
        .size;
    return Scaffold(
      body: Container(
        decoration: BoxDecoration(
          gradient: LinearGradient(
              begin: Alignment.topLeft,
              end: Alignment.bottomRight,
              colors: [Colors.tealAccent, Colors.lightBlue.shade900]
          ),
        ),
        width: double.infinity,
        height: size.height,
        child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Avatar(
                avatarUrl: currentUser?.avatarUrl,
                onTap: () {},
              ),
              SizedBox(height: 75),
              Container(
                  margin: EdgeInsets.symmetric(vertical: 10),
                  padding: EdgeInsets.symmetric(horizontal: 20, vertical: 5),
                  width: size.width * 0.8,
                  decoration: BoxDecoration(
                    color: OpacWhite,
                    borderRadius: BorderRadius.circular(29),
                  ),
                  child: TextFormField(
                      decoration: InputDecoration(
                          hintText: "Username",
                          border: InputBorder.none,
                          icon: Icon(
                            Icons.person,
                            color: DarkTurquoise,))
                  )
              ),
              Container(
                  margin: EdgeInsets.symmetric(vertical: 10),
                  width: size.width * 0.8,
                  child: ClipRRect(
                      borderRadius: BorderRadius.circular(29),
                      // ignore: deprecated_member_use
                      child: FlatButton(
                          padding: EdgeInsets.symmetric(
                              vertical: 18, horizontal: 40),
                          color: DarkTurquoise,
                          onPressed: () =>
                              Navigator.push(
                                context,
                                MaterialPageRoute(
                                  builder: (context) => NavScreen(),),
                              ),
                          child: Text(
                            "SAVE", style: TextStyle(color: SandYellow,
                            fontSize: 16,),
                          )
                      )
                  )
              )
            ]
        ),
      ),
    );
  }
}

My code in my User Model

class UserModel {
  String uid;
  String displayName;
  String avatarUrl;

  UserModel(this.uid, {required this.displayName, required this.avatarUrl});
}

finally I have this error: Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't. avatarUrl: currentUser?.avatarUrl,

This is the backend of Avatar();

import 'package:flutter/material.dart';

class Avatar extends StatelessWidget{
final String avatarUrl;
final VoidCallback onTap;

const Avatar({required this.avatarUrl, required this.onTap});

@override
Widget build(BuildContext context) {
  return GestureDetector(
    onTap: onTap,
        child: avatarUrl = null \\I have error there too, they want me to put late before string but it doesn't works
        ? CircleAvatar(
          backgroundImage: NetworkImage ("https://www.villascitemirabel.com/wp-content/uploads/2016/07/default-profile.png"),
          radius: 90.0,
          child: Icon(Icons.photo_camera, size: 45),
        )
            :CircleAvatar(
          radius: 50.0,
          backgroundImage: NetworkImage(avatarUrl)
        ),
    );
 }
}

Answer 1

UserModel currentUser;

Means that your variable is a non nullable UserModel. If you want to allow currentUser to be null,

UserModel? currentUser;

https://dart.dev/codelabs/dart-cheatsheet