How can I use a appear in another widget?

Im trying to use a appbar widget in another widget so my code is more clear. But the question is how can I do this ?

Heres my app bar widget :

class thisappbar extends StatelessWidget {
  thisappbar(this.isMe, this.username, {this.key});

  final Key key;
  final bool isMe;
  String username;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        elevation: 0,
        automaticallyImplyLeading: false,
        backgroundColor: Colors.white,
        flexibleSpace: SafeArea(
          child: Container(
            padding: EdgeInsets.only(right: 16),
            child: Row(
              children: <Widget>[
                IconButton(
                  onPressed: (){
                    Navigator.pop(context);
                  },
----------

And heres the way I wanna use it :

class ChatScreen extends StatelessWidget {

  static const route = '/messages';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(thisappbar(),
      ),
      body: Container(
          child: Column(
        children: <Widget>[
          Expanded(child: Messages()),
          NewMessage(),
        ],
      )),
    );
  }
}

Hope anyone can help thanks!

This is how my methods look now first the app bar widget after editing some changes from user @Steve Nosse :

class CustomAppBar extends StatelessWidget implements PreferredSizeWidget {
  CustomAppBar({this.key, this.isMe, this.username}) : super(key: key);

  final Key key;
  final bool isMe;
  String username;
  @override
  Widget build(BuildContext context) {
    /// Build you AppBar widget here
    return Scaffold(
      appBar: AppBar(
        elevation: 0,
        automaticallyImplyLeading: false,
        backgroundColor: Colors.white,
        flexibleSpace: SafeArea(
          child: Container(
            padding: EdgeInsets.only(right: 16),
            child: Row(
              children: <Widget>[
                IconButton(
                  onPressed: () {
                    Navigator.pop(context);
                  },
                  icon: Icon(Icons.arrow_back, color: Colors.black,),
                ),
                SizedBox(width: 2,),
                CircleAvatar(
                  backgroundImage: NetworkImage(
                      'https://randomuser.me/api/portraits/men/5.jpg'),
                  maxRadius: 20,
                ),
                SizedBox(width: 12,),
                Expanded(
                  child: Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      Text(
                        username, style: TextStyle(
                          fontSize: 16, fontWeight: FontWeight.w600),),
                      SizedBox(height: 6,),
                      Text("Online", style: TextStyle(
                          color: Colors.grey.shade600, fontSize: 13),),
                    ],
                  ),
                ),
                Icon(Icons.settings, color: Colors.black54,),
              ],
            ),
          ),
        ),
      ),


    );
  }

  @override
  Size get preferredSize => Size.fromHeight(56);
}

And heres where I use the appbar widget:

class ChatScreen extends StatelessWidget {

  static const route = '/messages';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: CustomAppBar(),
      body: Container(
          child: Column(
        children: <Widget>[
          Expanded(child: Messages()),
          NewMessage(),
        ],
      )),
    );
  }
}

And this is the error :

======== Exception caught by widgets library =======================================================
The following assertion was thrown building CustomAppBar(dirty):
A non-null String must be provided to a Text widget.
'package:flutter/src/widgets/text.dart':
Failed assertion: line 378 pos 10: 'data != null'

The relevant error-causing widget was: 
  CustomAppBar file:///Users/name/StudioProjects/project/lib/seitenleiste/nachrichten.dart:390:15
When the exception was thrown, this was the stack: 
#2      new Text (package:flutter/src/widgets/text.dart:378:10)
#3      CustomAppBar.build (package:project/seitenleiste/nachrichten.dart:357:23)
#4      StatelessElement.build (package:flutter/src/widgets/framework.dart:4646:28)
#5      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4572:15)
#6      Element.rebuild (package:flutter/src/widgets/framework.dart:4265:5)
...