Crash when minimizing OpenGL widget

I have created a very simple OpenGL application which displays a spinning textured cube. Everything works fine except that it crashes when minimizing the window. I've tracked down the error to the call to glClear(), but I've no idea why it's this function call which fails.
I've commented out several lines of code which do the actual rendering of the cube but the error still occurs. :(

cubewidget.h:

Code:

#ifndef CUBEWIDGET_H
#define CUBEWIDGET_H
 
#include <iostream>
#include <QtGui>
#include <QtOpenGl>
#include <cmath>
 
class cubeWidget : public QGLWidget
{
	Q_OBJECT
 
	public:
		cubeWidget();
 
	protected:
		void initializeGL();
		void paintGL();
		void resizeGL(int width, int height);
 
	public slots:
		void rotateOneStep();
 
	private:
		float rot;
		QImage texture;
		GLuint nr;
};
#endif

cubewidget.cpp:

Code:

#include "cubewidget.h"
 
cubeWidget::cubeWidget()
	: rot(0.0)
{
}
 
void cubeWidget::initializeGL()
{
	glClearColor(0.0, 0.0, 0.0, 0.0);
/*
	glEnable(GL_DEPTH_TEST);
	glEnable(GL_CULL_FACE);
	glEnable(GL_TEXTURE_2D);
 
	QString s = "C:/Dokumente und Einstellungen/me/Desktop/opengl_cmyk.png";
	if(!texture.load(s))
	{
		std::cout << "Error loading texture!" << std::endl;
		exit(1);
	}
	nr = bindTexture(texture, GL_TEXTURE_2D);
 
*/
   QTimer *timer = new QTimer(this);
   connect(timer, SIGNAL(timeout()), this, SLOT(rotateOneStep()));
   timer->start(10);
}
 
void cubeWidget::rotateOneStep()
{
	rot += 0.5;
	rot = rot < 360.0 ? rot : rot - 360.0;
	updateGL();
}
 
void cubeWidget::paintGL()
{
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
/*
   glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
 
   glRotatef(sin(rot      /180.0*3.141) * 180, 1, 0, 0);
   glRotatef(sin((rot+135)/180.0*3.141) * 180, 0, 1, 0);
   glRotatef(sin((rot+270)/180.0*3.141) * 180, 0, 0, 1);
 
	glBindTexture(GL_TEXTURE_2D, nr);
	glBegin(GL_QUADS);
		// vorne
		glTexCoord2d(0.0, 0.0);
		glVertex3i(-1, -1, +1);
		glTexCoord2d(1.0, 0.0);
		glVertex3i(+1, -1, +1);
		glTexCoord2d(1.0, 1.0);
		glVertex3i(+1, +1, +1);
		glTexCoord2d(0.0, 1.0);
		glVertex3i(-1, +1, +1);
 
		// rechts
		glTexCoord2d(0.0, 0.0);
		glVertex3i(+1, -1, +1);
		glTexCoord2d(1.0, 0.0);
		glVertex3i(+1, -1, -1);
		glTexCoord2d(1.0, 1.0);
		glVertex3i(+1, +1, -1);
		glTexCoord2d(0.0, 1.0);
		glVertex3i(+1, +1, +1);
 
		// hinten
		glTexCoord2d(0.0, 0.0);
		glVertex3i(+1, -1, -1);
		glTexCoord2d(1.0, 0.0);
		glVertex3i(-1, -1, -1);
		glTexCoord2d(1.0, 1.0);
		glVertex3i(-1, +1, -1);
		glTexCoord2d(0.0, 1.0);
		glVertex3i(+1, +1, -1);
 
		// links
		glTexCoord2d(0.0, 0.0);
		glVertex3i(-1, -1, -1);
		glTexCoord2d(1.0, 0.0);
		glVertex3i(-1, -1, +1);
		glTexCoord2d(1.0, 1.0);
		glVertex3i(-1, +1, +1);
		glTexCoord2d(0.0, 1.0);
		glVertex3i(-1, +1, -1);
 
		// oben
		glTexCoord2d(0.0, 0.0);
		glVertex3i(-1, +1, +1);
		glTexCoord2d(1.0, 0.0);
		glVertex3i(+1, +1, +1);
		glTexCoord2d(1.0, 1.0);
		glVertex3i(+1, +1, -1);
		glTexCoord2d(0.0, 1.0);
		glVertex3i(-1, +1, -1);
 
		// unten
		glTexCoord2d(0.0, 0.0);
		glVertex3i(-1, -1, -1);
		glTexCoord2d(1.0, 0.0);
		glVertex3i(+1, -1, -1);
		glTexCoord2d(1.0, 1.0);
		glVertex3i(+1, -1, +1);
		glTexCoord2d(0.0, 1.0);
		glVertex3i(-1, -1, +1);
	glEnd();*/
}
 
void cubeWidget::resizeGL(int width, int height)
{
	glViewport(0, 0, (GLint)width, (GLint)height);
 
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
   glOrtho(-2.0, 2.0, -2.0, 2.0, 2.0, -2.0);
}
 
int main(int argc, char **argv)
{
	QApplication app(argc, argv);
 
	cubeWidget cube;
	cube.show();
	return app.exec();
}

I'm using QT 4.1.4 for windows with MingW compiler(3.4.2)