Getting error invalid use of incomplete type ‘struct

[Basit781]

Hi to all,
Hope you all will be fine. Actually i wrote a simple class in which i used inheritance. But when i instantiate this class in main() i am getting error

invalid use of incomplete type ‘struct MainScreen'

I used the forward declaration and include the parent class header too in my main file but i am still getting the error. Here is my code

#include <QtGui/QApplication>
#include <QPalette>
#include "mainwindow.h"
#include <QtGui>
#include "Helper.h"
#include <qgridlayout.h>

class MainScreen;

int main(int argc, char *argv[])
{

    MainScreen * m = new MainScreen();   //error

}

MainScreen.cpp

#include <QtGui>

class MainScreen : public QGridLayout
{
public:
    MainScreen() {
       QToolButton *campaign_btn = new QToolButton();
       campaign_btn->setText("Germany");

      addWidget(campaign_btn,0,0,0);
    }
};

Please tell me why i am getting this error.

Thanks