Python Script with QGis - Python.exe Stopped Working

I purchased this book called Building Mapping Applications with QGIS and I am trying to work through one of the exercises. There is one script that I try to run that crashes python, generating the error message "python.exe has stopped working".

import sys
import os
from qgis.core import *
from qgis.gui import *
from PyQt4.QtGui import *
from PyQt4.QtCore import Qt



#############################################################################


class MapViewer(QMainWindow):
    def __init__(self, shapefile):
        QMainWindow.__init__(self)
        self.setWindowTitle("Map Viewer")

        canvas = QgsMapCanvas()
        canvas.useImageToRender(False)
        canvas.setCanvasColor(Qt.white)
        canvas.show()

        layer = QgsVectorLayer(shapefile, "layer1", "ogr")
        if not layer.isValid():
            raise IOError("Invalid shapefile")

        QgsMapLayerRegistry.instance().addMapLayer(layer)
        canvas.setExtent(layer.extent())
        canvas.setLayerSet([QgsMapCanvasLayer(layer)])

        layout = QVBoxLayout()
        layout.addWidget(canvas)

        contents = QWidget()
        contents.setLayout(layout)
        self.setCentralWidget(contents)

#############################################################################


def main():
    """  Our main program.
    """
    QgsApplication.setPrefixPath(os.environ['QGIS_PREFIX'], True)
    QgsApplication.initQgis()

    app = QApplication(sys.argv)

    viewer = MapViewer("C:/folder/shapefile.shp")
    viewer.show()

    app.exec_()

    QgsApplication.exitQgis()

#############################################################################

if __name__ == "__main__":
    main()

I don't know a whole lot about Python with QGIS so I'm not too sure what is causing python to crash. I am positive that all of the modules are importing correctly because if I define my paths and then import the modules in the script using the OSGeo4W Shell, there are no error messages.

This is how my paths are defined:

SET OSGEO4W_ROOT=C:OSGeo4W64
SET QGIS_PREFIX=%OSGEO4W_ROOT%appsqgis
SET PATH=%PATH%;%QGIS_PREFIX%bin
SET PYTHONPATH=%QGIS_PREFIX%python;%PYTHONPATH%

Given all of this, I think there has to be something wrong in the script. However, when I check the script using http://pep8online.com/ there are no errors that I can fix that will result in python not crashing.

Note that I have tried I have tried SET PATH=%QGIS_PREFIX%bin;%PATH% instead of SET PATH=%PATH%;%QGIS_PREFIX%bin with no success.