Circular Progress Indicator not being centered Flutter

I am integrating login in my app in which a circular progress indicator should appear while a request is being made. I don't know why the progressBar keeps appearing at the top instead of the center. Please help.

FadeAnimation(
        1.8,
        Center(
            child: Container(
          height: double.infinity,
          child: SingleChildScrollView(
            physics: AlwaysScrollableScrollPhysics(),
            padding: EdgeInsets.symmetric(
              horizontal: 40.0,
              vertical: 120.0,

            ),
            child: _isLoading
                ? Center(child: CircularProgressIndicator())
                : Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      Text(
                        'Sign In',
                        style: TextStyle(
                          color: Colors.white,
                          fontFamily: 'OpenSans',
                          fontSize: 30.0,
                          fontWeight: FontWeight.bold,
                        ),
                      ),
                      SizedBox(height: 30.0),
                      _buildEmailTF(),
                      SizedBox(
                        height: 30.0,
                      ),
                      _buildPasswordTF(),
                      _buildForgotPasswordBtn(),
                      _buildRememberMeCheckbox(),
                      _buildLoginBtn(),
                      _buildSignInWithText(),
                      _buildSocialBtnRow(),
                      _buildSignupBtn(),
                    ],
                  ),
          ),
        )),
      )

It because the height of SingleChildScrollView depend on vertical: 120.0 instead of the value container( height: double.infinity ) which mean the height of container is as big as it's parent.

The part:MediaQuery.of(context).size.height mean "as big as screen" you can use MediaQuery.of(context).size.height/2 instead to resize container's height

try this:

FadeAnimation(
    1.8,
    Center(
        child: _isLoading
            ? Container(
              width: MediaQuery.of(context).size.width,
              height: MediaQuery.of(context).size.height,
              child: Center(child: CircularProgressIndicator()))
            :Container(
              height: double.infinity,
              child: SingleChildScrollView(
                physics: AlwaysScrollableScrollPhysics(),
                padding: EdgeInsets.symmetric(
                  horizontal: 40.0,
                  vertical: 120.0,

            ),
            child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Text(
                    'Sign In',
                    style: TextStyle(
                      color: Colors.white,
                      fontFamily: 'OpenSans',
                      fontSize: 30.0,
                      fontWeight: FontWeight.bold,
                    ),
                  ),
                  SizedBox(height: 30.0),
                  _buildEmailTF(),
                  SizedBox(
                    height: 30.0,
                  ),
                  _buildPasswordTF(),
                  _buildForgotPasswordBtn(),
                  _buildRememberMeCheckbox(),
                  _buildLoginBtn(),
                  _buildSignInWithText(),
                  _buildSocialBtnRow(),
                  _buildSignupBtn(),
                ],
              ),
      ),
    )),
  )