SingleChildScrollView is not working on stack

Question

I'm new to flutter. In the bottom code, I've tried to wrap stack with SingleChildScrollView, but, even then, when I tap to type the keyboard covers the TextFormFields. The column's MainAxisSize is also set to minimum.Is this because of MediaQuery

Please help.

Note: In the below code there are three more containers of TextFormField

enum CircleAlignment {
  topLeft,
  // topRight,
  // bottomLeft,
  bottomRight,
}

MaterialApp(
      debugShowCheckedModeBanner: false,
      home: SafeArea(
        child: Scaffold(
          backgroundColor: Colors.white,
          body: Stack(
              children: [
              Positioned(
                top: MediaQuery.of(context).size.height / 9,
                child: Column(
                  children: [
                    Image.asset(
                      'assets/images/signup_2.jpg',
                      width: MediaQuery.of(context).size.width,
                      height: (8 * MediaQuery.of(context).size.height) / 19,
                    ),
                  ],
                ),
              ),
              SizedBox.expand(
                child: ClipRect(
                  child: CustomPaint(
                    painter: QuarterCirclePainter(
                      circleAlignment: CircleAlignment.topLeft,
                      color: Color(0xff1CEEEE)
                    ),
                  ),
                ),
              ),
              SizedBox.expand(
                child: ClipRect(
                  child: CustomPaint(
                    painter: QuarterCirclePainter(
                      circleAlignment: CircleAlignment.topLeft,
                      color: Colors.deepPurpleAccent
                    ),
                  ),
                ),
              ),
              SizedBox.expand(
                child: ClipRect(
                  child: CustomPaint(
                    painter: QuarterCirclePainter(
                      circleAlignment: CircleAlignment.bottomRight,
                      color: Color(0xff1CEEEE)
                    ),
                  ),
                ),
              ),
              SizedBox.expand(
                child: ClipRect(
                  child: CustomPaint(
                    painter: QuarterCirclePainter(
                      circleAlignment: CircleAlignment.bottomRight,
                      color: Colors.deepPurpleAccent
                    ),
                  ),
                ),
              ),
              Positioned(
                width: MediaQuery.of(context).size.width / 3,
                right: MediaQuery.of(context).size.width / 8,
                top: MediaQuery.of(context).size.height / 30,
                child: Text(
                  "Create new Account",
                  style: TextStyle(
                    fontSize: MediaQuery.of(context).size.width / 14,
                    fontWeight: FontWeight.w900,
                    fontFamily: 'Roboto',
                    color: Color(0xff000000),
                  ),
                ),
              ),
              Positioned.fill(
                left: MediaQuery.of(context).size.width / 14,
                right: MediaQuery.of(context).size.width / 14,
                top: (40 * MediaQuery.of(context).size.height) / 79,
                child: Column(
                    crossAxisAlignment: CrossAxisAlignment.center,
                    mainAxisSize: MainAxisSize.min,
                    children: [
                      Container(
                        height: MediaQuery.of(context).size.height / 10,
                        child: TextFormField(
                          decoration: InputDecoration(
                            contentPadding: EdgeInsets.only(
                              top: 0,
                              bottom: 0,
                              left: MediaQuery.of(context).size.width / 25,
                            ),
                            border: OutlineInputBorder(
                              borderRadius: BorderRadius.circular(15),
                              borderSide: BorderSide(
                                color: Color(0xff000000),
                                width: 2,
                              )
                            ),
                            enabledBorder: OutlineInputBorder(
                              borderRadius: BorderRadius.circular(15),
                              borderSide: BorderSide(
                                color: Color(0xff000000),
                                width: 2,
                              )
                            ),
                            focusedBorder: OutlineInputBorder(
                              borderRadius: BorderRadius.circular(15),
                              borderSide: BorderSide(
                                color: Color(0xff000000),
                                width: 2,
                              )
                            ),
                            hintText: "Full Name",
                            hintStyle: TextStyle(
                              color: Color(0xff000000),
                              fontFamily: 'Roboto',
                              fontWeight: FontWeight.w700,
                              fontSize: MediaQuery.of(context).size.width / 20,
                            )
                          ),
                        ),
                      )
                    ],
                  ),
                ),
              ],
            ),
          ),
        ),
    )