Qt + OpenGL + Graphics View Framework, crash with access 0x00000000

[czlc]

I create a new Qt application with OpenGL,just add scene in like this:

@{
ui->setupUi(this);

QGraphicsView *pSceneView = new QGraphicsView(this);
setCentralWidget(pSceneView);
QGLWidget *pGLWidget = new QGLWidget;
pSceneView->setViewport(pGLWidget);
pSceneView->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
pSceneView->setScene(new MainScene);
pGLWidget->makeCurrent();

QGLFunctions *glFuncs = new QGLFunctions(pGLWidget->context());
GLuint i;
glFuncs->glGenBuffers(1, &i);
glFuncs->glBindBuffer(GL_ARRAY_BUFFER, i); // unbind buffer cause crash
}@

the class MainScene is inherit from QGraphicsScene, just set SceneRect and rewrite drawBackground like this:
@MainScene::MainScene(QObject parent / = 0 */)
:QGraphicsScene(parent)
{
setSceneRect(-1024/2, -768/2, 1024, 768);
}

void MainScene::drawBackground(QPainter *painter, const QRectF &rect)
{
painter->drawRect(sceneRect());
}@

if i don't rewrite drawBackground or don't call glBindBuffer, it didn't crash。the Qt version is 5.3.2, os is windows 7。