Fix crash for pressing Ctrl+C in a message box

This fixes a regression introduced with e34dccc9e

Task-number: QTBUG-31635
Change-Id: I7c991f3b98f5c51e5fb3c12db04d3ace7be87010
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>

src/widgets/dialogs/qmessagebox.cpp

@@ -1392,7 +1392,7 @@ void QMessageBox::keyPressEvent(QKeyEvent *e)
 
 #if !defined(QT_NO_TEXTEDIT)
         if (e == QKeySequence::Copy) {
-            if (d->detailsText->isVisible() && d->detailsText->copy()) {
+            if (d->detailsText && d->detailsText->isVisible() && d->detailsText->copy()) {
                 e->setAccepted(true);
                 return;
             }