QTextDocumentLayoutPrivate::layoutFlow() accesses layout->lineAt(0), index out of range

There sometimes is a crash in (index 0 out of range)

qtextdocumentlayout.cpp::2497
QFixed height = QFixed::fromReal(layout->lineAt(0).height());

void QTextDocumentLayoutPrivate::layoutFlow(QTextFrame::Iterator it, QTextLayoutStruct *layoutStruct, int layoutFrom, int layoutTo, QFixed width)

My code calls mergeCharFormat(), this is the call stack

QTextLine::height() Line 1497 C++
> QTextDocumentLayoutPrivate::layoutFlow(QTextFrame::iterator it, QTextLayoutStruct * layoutStruct, int layoutFrom, int layoutTo, QFixed width) Line 2497 C++
QTextDocumentLayoutPrivate::layoutFrame(QTextFrame * f, int layoutFrom, int layoutTo, QFixed frameWidth, QFixed frameHeight, QFixed parentY) Line 2207 C++
QTextDocumentLayoutPrivate::layoutFrame(QTextFrame * f, int layoutFrom, int layoutTo, QFixed parentY) Line 2094 C++
QTextDocumentLayout::doLayout(int from, int oldLength, int length) Line 3001 C++
QTextDocumentLayout::documentChanged(int from, int oldLength, int length) Line 2964 C++
QTextDocumentPrivate::finishEdit() Line 1223 C++
QTextDocumentPrivate::endEditBlock() Line 1202 C++
QTextDocumentPrivate::setCharFormat(int pos, int length, const QTextCharFormat & newFormat, QTextDocumentPrivate::FormatChangeMode mode) Line 775 C++
QTextCursorPrivate::setCharFormat(const QTextCharFormat & _format, QTextDocumentPrivate::FormatChangeMode changeMode) Line 867 C++
QTextCursor::mergeCharFormat(const QTextCharFormat & modifier) Line 1943 C++
SMUHelpBrowser::emphasizeWordsInFindMode() Line 800 C++

This is the user code

void SMUHelpBrowser::emphasizeWordsInFindMode()
{
bool wholeWords{};
const QStringList words(m_Dialog->getWordsToFind(wholeWords));
QTextDocument* doc{document()};
if (words.isEmpty())

for (const auto word : words)
{
int numFound{};
for (QTextCursor csr(doc->begin()); !csr.isNull() && !csr.atEnd() && numFound < 15
{
QTextCharFormat format(csr.charFormat());
format.setForeground(Qt::red);
csr = doc->find(word, csr, wholeWords ? QTextDocument::FindWholeWords : QTextDocument::FindFlag(0));
if (!csr.isNull() && !csr.atEnd())
{
csr.movePosition(QTextCursor::WordRight, QTextCursor::KeepAnchor);
if (!csr.isNull() && !csr.atEnd() && csr.hasSelection())

numFound++;
}
}
}
}