Diagnostics page doesn’t check for dependency on php DOM support

I’m running Simply Static on a system that was recently upgraded from Ubuntu Server 18.04 to Ubuntu Server 20.04. The former defaulted to PHP 7.0 and the latter defaults to PHP 7.2. After the upgrade, Simply Static stopped working. The Simply Static Diagnostics page flagged cURL as missing, which was resolved by installing php7.2-curl. After that Simply Static still wasn’t working, but the Diagnostics page didn’t flag anything. The error was “Class ‘DOMDocument’ not found”. The fix was to install php7.2-xml.