GUI freeze after cancel QFileDialog

#!/usr/bin/python

from PyQt4.QtGui import *
from PyQt4.QtCore import *
import sys

class MyMainWindow(QWidget) :
    request = pyqtSignal()
    def __init__(self, parent = None):
        super(QWidget, self).__init__(parent)
        self.button = QPushButton('Save File')
        layout = QVBoxLayout()
        lineedit = QLineEdit()
        layout.addWidget(self.button)
        layout.addWidget(lineedit)
        self.setLayout(layout)
        self.button.clicked.connect(self.buttonClicked)

    def buttonClicked(self):
        strn = QFileDialog.getSaveFileName(self, "Caption")

if __name__ == '__main__':
    app = QApplication(sys.argv)
    w = MyMainWindow()
    w.show()
    sys.exit(app.exec_())