Thank you very much for this suggestion! I tested it and it works nicely. I will implement it in my notebook, maybe it will be less prone to disappearing!

Thanks, that explains half of the mystery (at least at my level of understanding...) So the code is not there, but the drop-down menu generated by the code, which performs the reading of the data of the selected reactor core, is still there. Brilliant, thank you very much!

What I still need to understand is why the code disappeared, and how I can protect the notebook that if I copy the snippet back as you described and as I did it in the past, it does not disappear again. This would be a very appreciated piece of information.

Thank you for your comment. Yes, I was also thinking of this possibility. In fact, quite some time ago (the error appeared several months ago), I did exactly what you suggest. It worked for a while, but the same symptom reappeared after a few days. I wonder if it happens to you as well.

And what is even more mysterious is that the code must still be there, because when I run the version with the "Null", the drop-down menu still supplies the requested data. You can try it by downloading the notebook from the Notebook archive and running it.

Thank you very much for devoting more attention to this. I apologise for being difficult, but this is still not what I want. It is not the results of an operation/command which I am looking for, rather for the piece of code which creates the drop-down menu and the reading commands.

I can illustrate it by using an earlier version of the Notebook, where I can still make the content of the cell visible. In that older version, when I open the notebook, still the same drop-down menu is visible as in the recent code (that you also run), but the code generating it is hidden:

Now to make the code visible, I click on the vertical line second from the left at the right margin (the one which has a semi-arrow on the top), to make the rest of the cell visible, and then it looks like this:

This is what I cannot achieve with the current Notebook, i.e. to make the piiece of code visitble above the drop-down menu. The vertical lines on the right hand side margin look different (see the included figure in my original post, i.e. on the top of this conversation). And if I click on the right hand side line (which is marked blue in the figure), then, instead of the piece of code shown in the last figure above, only the text "Null" appears (again, this is also shown in the first figure).

But the piece of code is still in my Notebook, since the drop-down menu still works, but I cannot make it visible. I want this piece of code to be possible to make visible, because this is from where the users can find out the data of the GitHub repository, if they want to upload further data (as I ask them in the book, which refers to the Notebook in the Repository.

I would appreciate very much if you could help me to make the code "snippet" visible.

Thank you for looking at this. The post you show confirms that the cell works, because your notebook successfully read the data corresponding to the SFR (Sodium Fast Reactor) core.

My problem is that I cannot make the content of the cell visible, which performs the reading through the choice in the drop-down menu.