'class QCoreApplication' has no member named 'setStyleSheet'

[twewm]

I simply want to use a style sheet.
I get this error when I want to Compile

C:\Erfan\C++\Exersize\QT\untitled01\main.cpp:19: error: 'class QCoreApplication' has no member named 'setStyleSheet'
QApplication::instance()->setStyleSheet(ts.readAll());
^

Here's my Code

#include "mainwindow.h"
#include <QApplication>
#include <QFile>
#include <Qtextstream>
 
 
int main(int argc, char *argv[])
{
 
    QFile f(":qdarkstyle/style.qss");
    if (!f.exists())
    {
        printf("Unable to set stylesheet, file not found\n");
    }
    else
    {
        f.open(QFile::ReadOnly | QFile::Text);
        QTextStream ts(&f);
        QApplication::instance()->setStyleSheet(ts.readAll());
    }
 
    QApplication a(argc, argv);
    MainWindow w;
 
    w.show();
 
    return a.exec();
}

#include "mainwindow.h"
#include <QApplication>
#include <QFile>
#include <Qtextstream>


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

    QFile f(":qdarkstyle/style.qss");
    if (!f.exists())
    {
        printf("Unable to set stylesheet, file not found\n");
    }
    else
    {
        f.open(QFile::ReadOnly | QFile::Text);
        QTextStream ts(&f);
        QApplication::instance()->setStyleSheet(ts.readAll());
    }

    QApplication a(argc, argv);
    MainWindow w;

    w.show();

    return a.exec();
}

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