在FexibleSpaceBar中禁用了按钮

Question

我在FlexibleSpaceBar添加按钮时遇到问题。出于某种原因，它总是被禁用。我已经尝试了FlatButton和RaisedButton，但两者都出现了同样的问题。如果我将它放在我页面上的其他位置，该按钮可以正常工作 这是代码：

  new SliverAppBar(
    backgroundColor: new Color.fromRGBO(253, 61, 98, 0.9),
    floating: false,
    expandedHeight: _appBarHeight,
    actions: <Widget>[
      new IconButton(
        icon: new Icon(Icons.search),
        onPressed: () => Navigator.of(context).push(new MaterialPageRoute(builder: (BuildContext context) => new FriendSearchPage())),
      ),
      new IconButton(icon: new Icon(Icons.settings), onPressed: null)
    ],
    pinned: true,
    leading: new Icon(Icons.person),
    title: new Text("Your Profile"),
    flexibleSpace: new FlexibleSpaceBar(
      background: new Stack(
        fit: StackFit.expand,
        children: <Widget>[
          new Center(
            child: new Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                new Padding(padding: new EdgeInsets.only(top: 50.0),),
                new Container(
                  height: 100.0,
                  width: 100.0,
                  decoration: new BoxDecoration(
                    borderRadius: new BorderRadius.circular(100.0),
                    border: new Border.all(color: Colors.white70)
                  ),
                  child: new Icon(Icons.person, size: 70.0, color: Colors.white,),
                ),
                new Padding(padding: new EdgeInsets.only(top: 20.0)),
                new Text(_firstName + " " +_lastName, style: new TextStyle(color: Colors.white),),
                new Padding(padding: new EdgeInsets.only(top: 20.0)),
                ////////////////////////////////////////////////////////
                new FlatButton( // !!!! Disabled? !!!!
                  child: new Text("Copy Friend ID"),
                  onPressed: () {
                    showDialog(
                      context: context,
                      child: new Text("ok")
                    );
                  }
                )
              ],
            ),
          ),
          new Stack(
            children: <Widget>[
              new Container(
                decoration: new BoxDecoration(
                    gradient: new LinearGradient(
                      colors: <Color>[
                        new Color.fromRGBO(253, 61, 98, 0.4),
                        new Color.fromRGBO(253, 94, 65, 0.2)
                      ],
                      begin: const Alignment(0.0, 0.6),
                      end: const Alignment(0.0, -1.0),
                    )
                ),
              ),
            ],
          )
        ],
      ),
    ),
  ),