Im asking as if you use really old Qt version, then upgrading Qt version
might need some fixes in the code.

If its not so old, then often it just works with newer version.

Also your Creator is pretty old and the newer versions did bring good things and
might be better at closing. At least I have not seen any issues and i use multiple Creators often.

But i wanted to know your Qt version you are using as I have had a bit issues
with old Qt version and have a brand new Creator detecting it.

The reason there is talk about upgrading your OS, is that it has newer versions that one can apt-get.

However, it might work just installing a newer one (Creator) , but you might get issues that some OS system files is not the expected versions
and then the fun begins.

I like to try this , however it is missing the stating point - which menu ?

Registering Installed Qt Versions

Qt Creator automatically detects the Qt versions that are registered by your system or by the Qt Installer. To view detailed information for each Qt version, 

**select it in the list and select Details in the Qt version for section** 

???  

I appreciate all the well meaning suggestion to "upgrade" both of my OS and QtCreator , but my OS is not broken.

I can check if Ubuntu has install process for newer version...

You QtCreator version is really outdated!
I have installed Ubuntu 18.04, which is a quit old LTS and there the current Qt Creator version is 4.5.2!

So I think you should start first updating your linux installation

(I use the precompiled Qt which came with it (from apt-get if necessary), which seems to be 5.12.8. But that's just for reference, it's not a Creator version.)

For Qt Creator, I am 4.11.0 (apparently from 2019). I cannot quite recall whether I got it from apt-get or from the Qt site/Ubuntu software updater. But it was whatever was current 6+ months ago.

So your 3.5.1 looks pretty old. I don't update to latest stuff, but yours from 5+ years ago I would move on from. Where/when did you get it? What Ubuntu version are you?

But what Qt version are you using ?
You can see that in "Projects" side button when you have project open

I can check if Ubuntu has install process for newer version...

I like to be realistic - I do not want to screw around updating my hammer if the "problem " is that the handle could use fresh paint job.

Seriously - can I update and KEEP the stuff I have added during my coding ?

Likes "bluetooth" module, projects files structure etc.
.

Did you check the Stop before build option in your settings like described here ?