First I used a persistent live USB of 18.04 and the software-boutique worked only for once.
Afterwards it opens but it does not load anything, just remains a blank window.
The software-boutique is part of the Welcome application. The Welcome application loads normally, but if you click to the software button to open the Boutique, you will receive a blank screen again.
Then I installed ubuntu-mate on an external drive and it happened the same. Software-boutique was loaded for a couple of times and I was capable to install some software. Then it was not functioning anymore.
Later I deleted and reinstalled ubuntu-mate on the external drive and again Software boutique worked only for once.
Then I removed and reinstalled the Welcome and Software Boutique packages, but without result.
I did the reinstallation at least 4 times successfully, but Software Boutique never worked.
Then why should I keep a program which does not work?
So I completely removed both packages, Welcome and Boutique.
We can always install a new program by downloading a deb package, or using the
apt-get install and snap install commands.
The only reason that I miss the Software Boutique is that it give us the opportunity to preview a new program and it presents us with a list of new applications that we are not always aware of.