AI. FILE ICON? Missing when saving an ai. file

Hello to amsftf11814373, i know its too late for the answer, but it may help others.
I have fixed this problem via FileTypesMan - Alternative to 'File Types' manager of Windows (nirsoft.net) 
1. Download the ZIP file located at the bottom of the page and proceed to install it.
2. look for the target ext (.ai in this case) and double-click on it

3. on newly opened dialog click on the button adjacent to "Default icon."

4. Choose an icon of your preference, then click 'OK.'

Done!

Yes. Thank you - this DID start with ai. files showing an XD icon as well as opening in XD. I did manage to work around that and open as an ai. but, as mentioned, the icon turned "blank". It also seems related to a registry glitch where the NEWest version of Illustrator 2019 is actually a version 23 and the registry showing it as 22 (and this may be a Windows 10 glitch). Overall though - you're answer here is right on target.

This happens based on file associations. They have gone berserk for many people after the CC2019 update.

You might try threads about files opening in the wrong application such as this one: My Illustrator files opens as XD files and I can't change it