Description:
While EDITING or FORMATTING certain ODS documents only (also when these same documents are in XLS or XLSX format), Calc crashes at different times and while doing different operations, always popping up an alert 

Libre Office 7.0 - Fatal Error 
Invalid child index

The problem is present for 4-5 years now, but it doesn't happen too often (I don't use these specific documents too often).

I can't recognize any trigger pattern. Sometimes it crashes when I directly double-click into a cell (to edit the content of that cell) if that window didn't have focus before double-clicking it. But not always. Sometimes it crashes when I change font size of a cell, but not always. Sometimes it crashes when I hit cmd+s (it crashes before saving the document), but not always. 

I'm attaching one document that has this problem. I also can't say for sure what is it in this document that causes the problem. I thought that maybe an original document that I created was corrupt in some way, and I always used it as a template for the other documents later. So recently I recreated a fresh document from scratch, but the problem is still present. So, it must be about some specific formatting or some specific content inside.

What I can think of as being different than other spreadsheets I use, is:

- the font used is Geneva

- there is a large number of rows of great height (a lot of text in one of the columns) - usually there are about a hundred to several hundred rows all toghether, and maybe about 20% of them of great height

- inside cell contents, a hard line break (cmd+enter) is extensively used

- inside cell contents with a lot of text (very high rows), there is a lot of text formatting - like using bold text and different font sizes inside the same cell

- page header has multiple lines

Steps to Reproduce:
1. Open document
2. Try editing and formating the contents (sometimes everything works fine for hours)
3. Calc crashes out of the blue on some usual editing or formatting operation

Actual Results:
Calc crashed, everything since last save is lost

Expected Results:
Calc not crashing, editing works fine. :-)


Reproducible: Sometimes


User Profile Reset: Yes



Additional Info:
Copy&paste from About window doesn't work. See screenshot attached for version information.
I alse tested the same document attached here under Libre Office Safe mode. It crashed after 3-4 minutes of editing, when I clicked on Print Preview tool button.