networkImage error handling/placeholder not working

How can i add a network handling/placeholder in networkimage when the url return null?

  body: Center(
        child: ListView.builder(
            itemCount: users.length,
            itemBuilder: (BuildContext context, int index) {
              User user = users[index];
              return ListTile(
                  title: Text(user.firstName),
                
                  subtitle: Text(user.role),
                  leading: CircleAvatar(
                    backgroundImage: NetworkImage(user.avatar.toString()),
                     errorWidget: (context, url, error) =>  //this part not working
                     Icon(Icons.error),
                  ),
                  onTap: () {});
            })));

>Solution :

Use following code.

body: Center(
        child: ListView.builder(
            itemCount: users.length,
            itemBuilder: (BuildContext context, int index) {
              User user = users[index];
              return ListTile(
                  title: Text(user.firstName),
                
                  subtitle: Text(user.role),
                  leading: CircleAvatar(
                    backgroundImage: Image.network('ImageURL....',
                            errorBuilder: (context, exception, stackTrace) {
                              return const Icon(Icons.error);
                            },
                          ), 
                  onTap: () {});
            })));