Missing indexes after offline OS install cause false "package does not exist" error.

[black-puppydog]

This just happened during setup with a family member:

• We installed ubuntu mate as usual via graphical installer, but without internet connection.
• After installation we opened the software boutique and searched for "telegram". This brought up the correct package.
• When checking out, the software boutique just showed the error "Package does not exist" (or sth similar, going from memory here)

After manually running a sudo apt update we re-tried installation via the software boutique and this time it worked. So I guess that the missing/out-of-date indexes were the only issue here.

If the boutique could be made aware of this and offer automatic resolution that'd be cool. :)