while (nextParent == commonParent) {
        if (ansList.size() < bestParentRow + 1)
            // If the list is longer, it means that at the end of it is a descendant of the new index.
            // We insert the ancestors items first in that case.
            break;

        const QModelIndex nextSibling = ansList.value(bestParentRow + 1);

        if (!nextSibling.isValid()) {
            continue;
        }

I ran into this when I added a fourth mail repository to KMail.  The code loops from line 185 to line 194; nothing in between affects the condition of line 185.  In effect, KMail hangs on startup and I am unable to access its preferences.