Out of the blue run I into this issue.
The only slightly unusual before was, that I had a confirmation box regarding an unsaved file while quit the session. Then, when I want to restart the same session, it crached always.
If you like I can submit the BT, but think it's not needed to approve this patch.

Hmm, the internal state is really broken if that doesn't work.
I don't think this check will really solve the issue.
Had you any chance to see a backtrace with e.g. the index?

Hmm, perhaps the restore view stuff does create the issues:

// restore Document lru list so that all tabs from the last session reappear
 const QStringList lruList = group.readEntry("Documents", QStringList());
 for (int i = 0; i < lruList.size(); ++i) {
     auto doc = KateApp::self()->documentManager()->findDocument(QUrl(lruList[i]));
     if (doc) {
         const int index = m_lruDocList.indexOf(doc);
         if (index < 0) {
             registerDocument(doc);
             Q_ASSERT(m_lruDocList.contains(doc));
         } else {
             m_lruDocList.removeAt(index);
             m_lruDocList.append(doc);
         }
     }
 }

the else case with the lruDocList changes might mess up some lru => tab order, or?

I would propose this fix:

hm, can't say much helpful to your patch. Send you my BT.
But from my point of view is this assert here redundant. When you dislike my check, you can drop the assert anyway.

I'm sorry, I couldn't reproduce the crash to verify your patch. But at least cause it on the first sight no new crash here :-)