Comment 4 for bug 1577237

Probably no need to add a new bug report since there have been many similar ones due to the old version of Pinta and Mono that Ubuntu still ships.
Upgrading to the latest version of Mono has sometimes fixed the crashes (https://www.mono-project.com/download/stable/#download-lin)

However, my recommendation is to upgrade to Pinta 2.1 either by using the snap or flatpak packages, or by installing the PPA from https://xtradeb.net/apps/pinta/ which recently got Pinta 2.1 packaged as a .deb