Cannot use QGuiApplication to create GUI app: Cannot create a QWidget when no GUI is being used

running the following test case gives error message: "QWidget: Cannot create a QWidget when no GUI is being used
Aborted (core dumped)
"
But don't see the reason from the doc.

#include <QGuiApplication>
#include <QWidget>
int main( int argc, char* argv[])
{
QGuiApplication a(argc,argv);
QWidget w;
w.show();
return a.exec();
}