Flutter – How to remove below line under appBar despite elevation set to 0

I want to remove following blue Line underneath my appBar.

What have I tried already?

1. changing color of appBar backgroundColor: kTextfieldBorderColor,

2. I have set elevation to 0 elevation: 0

3. Reviewing this similar query here but it doesn’t give any satisfactory answer but only a temporary solution. edit: I tried the solution mentioned here and it doesn’t work by making both Scaffold and appBar transparent

4. I also tried adding toolbarHeight: kToolbarHeight + 1.25, to appBar and this even doesn’t work.

Below am adding the complete Scaffold code, for you to get an idea why its causing this problem.

Widget build(BuildContext context) {
return Scaffold(
  backgroundColor: kBackgroundColor,
  appBar: AppBar(
    backgroundColor: kTextfieldBorderColor,
    elevation: 0,
    // leading: null,
    actions: <Widget>[
      IconButton(
          icon: Icon(Icons.close),
          onPressed: () {
            //Implement logout functionality
            _auth.signOut();
            Navigator.pop(context);
          }),
    ],
    title: Text('⚡️Chat'),
  ),
  body: SafeArea(
    child: Column(
      mainAxisAlignment: MainAxisAlignment.spaceBetween,
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: <Widget>[
        Container(
          decoration: kMessageContainerDecoration,
          child: Row(
            crossAxisAlignment: CrossAxisAlignment.center,
            children: <Widget>[
              Expanded(
                child: TextField(
                  onChanged: (value) {
                    //Do something with the user input.
                  },
                  decoration: kMessageTextFieldDecoration,
                  cursorColor: kTextfieldBorderColor,
                ),
              ),
              TextButton(
                onPressed: () {
                  //Implement send functionality.
                },
                child: const Text(
                  'Send',
                  style: kSendButtonTextStyle,
                ),
              ),
            ],
          ),
        ),
      ],
    ),
  ),
);

Code of constants used for color, textField etc

const kBackgroundColor = Color(0xffEDD9C8);
const kRoundedButtonColor = Color(0xffC68E5D);
const kTextfieldBorderColor = Color(0xffC68E5D);
const kMessageTextFieldDecoration = InputDecoration(
  contentPadding: EdgeInsets.symmetric(vertical: 10.0, horizontal: 20.0),
  hintText: 'Type your message here...',
  border: InputBorder.none,
);

const kMessageContainerDecoration = BoxDecoration(
  border: Border(
    top: BorderSide(color: Colors.lightBlueAccent, width: 2.0),
  ),
);