In order to have working app-center again, you will have to uninstall the flatpak bundles that you might have installed earlier (This is temporary, until we ship a fix in next OS update).
Open Terminal
flatpak uninstall -y com.rstudio.RStudio
flatpak uninstall -y com.microsoft.winfile
Restart system and see if app-center works. Let us know.
Thanks.