SliverAppBar hides the widgets in SliverFillRemaining

I have some widgets in SliverFillRemainingWhen Scrolling the page, the widgets are hidden below the SliverAppBar.

I want to see the widgets below the AppBar.

return new Scaffold(
  body: new Stack(
    children: <Widget>[
      new CustomScrollView(
        controller: _scrollController,
        slivers: [
          new SliverAppBar(
            expandedHeight: 256.0,
            pinned: true,
            snap: true,
            floating: true,
            forceElevated: true,
            flexibleSpace: new FlexibleSpaceBar(
              title: new Text("Status Entry"),
              centerTitle: true,
              background: new Image.asset(
                "assets/images/noimage.png",
                fit: BoxFit.cover,
              ),
            ),
          ),

    SliverFillRemaining(
            child: Center(
              child: new SingleChildScrollView(
                  padding: const EdgeInsets.only(
                      left: 20.0, right: 20.0, top: 0.0, bottom: 0.0),
                  child: new Column(
                    //mainAxisAlignment: MainAxisAlignment.start,
                    mainAxisSize: MainAxisSize.max,
                    children: <Widget>[
                      TextField(
                        enabled: false, // will disable paste operation
                        decoration: InputDecoration(labelText: 'Work name'),
                      ),
                      TextFormField(
                        decoration: InputDecoration(
                            labelText: 'Enter your current status'),
                      ),

                    ],
                  )),
            ),
          ),

My output is like this:

enter image description here