it seams your divider has no width, you can add it to the Container:

SizedBox(
  height: 10,
  child: Center(
    child: Container(
      margin: EdgeInsetsDirectional.only(start: 1.0, end: 1.0),
      height: 5.0,
      width: double.infinity,
      color: Colors.red,
    ),
  ),
),