Ubuntu 20.04 Unity No applications in "start" menu after upgrading from 18.04

Question

So yesterday I've upgraded to Ubuntu 20.04 from 18.04, firstly I had to finish up with a partial upgrade that the software updater suggested, then it asked me to remove all the unused packages so I did, then I've ran it one more time to be sure ALL the software is up to date, it seemed at first glance it went well.
Today I found out there are no applications in the "start" menu (when you click on the Ubuntu icon on the launcher, some may call it applications lens) - I only got a message "Sorry, there is nothing that matches your search" application menu gone too with same message.

I did search a bit but I found a similar problem only for gnome caused by gnome-tweaks tool, but I am using Unity, but in the end...

...I found the solution - see steps in my answer :)

Answer 1

Since I know I did use a tweak tool, although not for gnome, but for Unity - unity-tweak-tool I did try to make a similar approach and it worked :) Also I did want to keep my Unity, not replace it with another OS GUI (which also works btw - switching to e.g. Gnome BEFORE you login).

but first ... (Step 1)

First I found out that I don't have a GUI way to launch terminal, luckily, there is a shortcut
Open your terminal: press CtrlAltT

Step 2: trying to run unity-tweak-tool

Try to run unity-tweak-tool from terminal if the graphical interface of unity-tweak-tool starts fine - skip to step 3
However, for me it yelled about missing packages, it seemed all to be related to unity-lens-SOMETHING, when I installed one, another popped out, so I just ended up installing all of the unity-lens-*, error could look like
Error: schema com.canonical.Unity.ApplicationsLens not installed - notice the pattern of this error and what I'm installing:

sudo apt install unity-lens-applications
sudo apt install unity-lens-music
sudo apt install unity-lens-video
sudo apt install unity-lens-files
sudo apt install unity-lens-photos

After this, run unity-tweak-tool from terminal again and it should start normally, if it does => go to step 3, if it does not, try to install any dependencies it requires - usually the package has a similar name as the required stuff in com.canonical.Unity.SOMETHINGMISSING something unity-*.
If you just write DONT ENTER YET! sudo apt install unity- to the terminal and hit TAB 2x, it will show you all unity-* packages - that is not THAT much of them, so you can search there for any missing stuff, similarly for other missing stuff.

Step 3: reseting Unity tweaks

Now that you are in a working GUI of unity-tweak-tool, under the "Unity" category, open Launcher - write down all the settings you made there and hit the Restore defaults button, confirm any questions (you can set your settings back later) - do the same for all the tabs in the Unity category:

Unity -> Launcher -> Restore defaults
Unity -> Search -> Restore defaults
Unity -> Panel -> Restore defaults
Unity -> Switcher -> Restore defaults
Unity -> Additional -> Restore defaults

Step 4: Restart your computer

After restarting, your start menu and the whole panel will be reset to its original position and everything (we've turned off all the tweaks), but application & search menu should now work normally.

Step 5: Go to unity-tweak-tool again and adjust any settings back to your custom