Segmentation Fault when saving a QImage in Qt 4.6

This function should save its QImage image to a file either specified by the user or that it was opened from.

void ImageDocument::save(QString file)
{

if (file.isEmpty())

file = filename;

cout << qPrintable(file) << endl;
if (this->image.save(file, "PNG")) {

if (!this->undoStack->isClean())

this->undoStack->setClean();

filename = file;

}

}

If I were to use it with the following, it would cause a segmentation fault:

void PixelEasel::saveAs()
{

QString fileName(QFileDialog::getSaveFileName(this, tr("Save As"),

QDir::currentPath(),
tr("Images (*.png *.gif)")));

if (fileName.isEmpty())

return;

activeDocument()->save(fileName);
saveAct->setEnabled(!undoGroup->isClean());

}

Whereas the exact same code, except for the manual specification of the file path, will succeed.

Apparently, I am a moron and wrongly assumed that QMdiArea::activeSubWindow() will give you the sub-window if the area of focus is the top-level window menu. (Hint: QMdiArea::currentSubWindow() will do the trick). It is apparent I have not been using C++ in such a long time, because Qt was letting me execute methods on an object that was cast from a NULL pointer. I thought that would throw an exception earlier rather than when it went and tried to look for class members that didn't officially exist.