How to add Floating action Button in Bottom Naviga

I am trying to add a Floating Action Button in the middle of the Bottom Navigation bar. Problem is border is not appearing and also margin in Floating Action Button and icons not according to my requirement.

Here is a picture of the issue. Achieved Image

Here is a picture what I want Required Image

Code

bottomNavigationBar: SafeArea(child: _buildBottomBar(context)),
    floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
    floatingActionButton: CircularGradientButton(
      tooltip: 'Increment',
      child: new Image.asset(UIData.cameraIcon),
      gradient: LinearGradient(colors: <Color>[
        Color.fromARGB(255, 17, 153, 142),
        Color.fromARGB(255, 56, 239, 125)
      ]),
      callback: () => openCamera(),
      //_optionsDialogBox(),
      elevation: 2.0,
    ), 



Widget _buildBottomBar(BuildContext context) {
return Material(
  color: Colors.white,
  child: TabBar(
    isScrollable: false,
    unselectedLabelColor: Colors.white.withOpacity(0.3),
    indicatorColor: Colors.white,
    tabs: <Tab>[
      new Tab(
        // set icon to the tab
        icon: Image.asset(
          _tabController.index == 0 ? UIData.mapIconActive : UIData.mapIcon,
          width: 30,
          height: 30,
        ),
      ),
      new Tab(
        icon: Image.asset(
          _tabController.index == 1
              ? UIData.listingIconActive
              : UIData.listingIcon,
          width: 30,
          height: 30,
        ),
      ),
      new Tab(
        icon: Image.asset(
          _tabController.index == 2
              ? UIData.messageIconActive
              : UIData.messageIcon,
          width: 30,
          height: 30,
        ),
      ),
      new Tab(
        icon: Image.asset(
          _tabController.index == 3
              ? UIData.settingsIconActive
              : UIData.settingsIcon,
          width: 30,
          height: 30,
        ),
      ),
    ],
    controller: _tabController,
  ),
);

}

There are many possible solutions, one of them is to add padding and border.

import 'package:charts_flutter/flutter.dart' as prefix0;
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        backgroundColor: Colors.blueAccent,
        floatingActionButton: Padding(
          padding: EdgeInsets.only(top: 20),
          child: SizedBox(
            height: 70,
            width: 70,
            child: FloatingActionButton(
              backgroundColor: Colors.transparent,
              elevation: 0,
              onPressed: () {},
              child: Container(
                height: 70,
                width: 70,
                decoration: BoxDecoration(
                  border: Border.all(color: Colors.white, width: 4),
                  shape: BoxShape.circle,
                  gradient: LinearGradient(
                    begin: const Alignment(0.7, -0.5),
                    end: const Alignment(0.6, 0.5),
                    colors: [
                      Color(0xFF53a78c),
                      Color(0xFF70d88b),
                    ],
                  ),
                ),
                child: Icon(Icons.photo_camera, size: 30),
              ),
            ),
          ),
        ),
        floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
        bottomNavigationBar: BottomAppBar(
          color: Colors.white,
          child: Container(
            height: 80,
            color: Colors.white,
          ),
        ),
      ),
    );
  }
}