堆栈中的窗口小部件分开而不是重叠

代码:

                  SizedBox(
//                    height: 120,
                    child: Stack(
                      children: [
                        Image(
                          image: NetworkImage(campaign.getHeaderImage()),
                          fit:BoxFit.cover,
                        ),
//                      Image.network(campaign.getHeaderImage()),
                        Container(
                          padding: EdgeInsets.all(2),
//                          alignment: Alignment.bottomCenter,
                          decoration: BoxDecoration(
                            gradient: LinearGradient(
                              begin: Alignment.topCenter,
                              end: Alignment.bottomCenter,
                              colors: <Color>[
                                Colors.black.withAlpha(0),
                                Colors.black12,
                                Colors.black45
                              ],
                            ),
                          ),
                          child: PageHeader(
                            title: campaign.isPast()?"Past Campaign Actions":"Actions",
                            backButton: true,
                            textColor: Colors.white,
                          ),
                        )
                      ],
                    ),
                  ),

Image of The above code

On adding height:120 to either the sized box or to the image i am getting this result : Image after adding height constraint