The method 'call' was called on null. Flutter

I'm building a calculator in flutter, I am trying to pass a callback onclick function with a parameter to the button widget which is in a different file but when I click any button it throws an exception that method was called on null. Also I dont know how to declare a function with a parameter in the CustomBtn class.

This is the main widget where I pass the function:

CustomBtn(
              btext: '8',
              color: Colors.grey[600],
              textColor: Colors.grey[50],
              onClick: buttonPressed('8'),
            ),

This is the button widget:

  class CustomBtn extends StatelessWidget {
  final String btext;
  final color;
  final textColor;
  final Function onClick;

  CustomBtn({
    this.btext,
    this.color,
    this.textColor,
    this.onClick,
  });
  @override
  Widget build(BuildContext context) {
    return RaisedButton(
      child: Text(
        btext,
        style: TextStyle(fontSize: 35.0, color: textColor),
      ),
      onPressed: () => onClick(btext),
      color: color,
      padding: EdgeInsets.fromLTRB(0.0, 24.0, 0.0, 24.0),
    );
  }
}