How to remove bottom appbar border

So i just made sliverappbar with tabbar everything is fine until this border bottom appear from no where, as you can see in the image there is a thin line between tabbar and tabbarview, i want to get rid of it, i thought it was elevation but i already set it to 0 aand it's still there

Here is the appbar code

SliverAppBar(
      iconTheme: IconThemeData(color: Colors.white),
      titleSpacing: 18.w,
      backgroundColor: Const.colorPrimary,
      elevation: 0,
      automaticallyImplyLeading: false,
      title: Text(
        'Star Movie',
        style: Const.textPrimary,
      ),
      actions: [
        IconButton(
          constraints: BoxConstraints(),
          padding: EdgeInsets.zero,
          onPressed: () {},
          icon: Image.asset(
            'assets/icon_search.png',
            width: 24.w,
          ),
        ),
        SizedBox(width: 18.w),
      ],
      bottom: PreferredSize(
        preferredSize: new Size(80.w, 80.h),
        child: Container(
          color: Const.colorPrimary,
          padding: EdgeInsets.fromLTRB(0.w, 16.h, 0.w, 16.h),
          child: Container(
            height: 48.h,
            margin: EdgeInsets.fromLTRB(18.w, 0, 18.w, 0),
            padding: EdgeInsets.fromLTRB(4.w, 4.h, 4.w, 4.h),
            decoration: BoxDecoration(
              border: Border.all(color: Const.colorIndicatorBorder),
              borderRadius: BorderRadius.circular(50.r),
            ),
            child: TabBar(
              indicator: ShapeDecoration(
                shape: StadiumBorder(),
                color: Const.colorIndicator,
              ),
              labelStyle: Const.textPrimary.copyWith(fontSize: 14.sp),
              tabs: [
                Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: [
                    Image.asset(
                      'assets/icon_now_showing.png',
                      width: 24.w,
                    ),
                    Tab(text: 'Now Showing'),
                  ],
                ),
                Tab(text: 'Coming Soon'),
              ],
            ),
          ),
        ),
      ),
    );

You can remove these thick lines by calling border with providing width=0/color=containerColor.

 Container(
  decoration: BoxDecoration(
  border: Border.all(color: yourBackgroundColor, width:0),
   ),