Mail is Unable to Open a URL
According to your error message it looks like a Mail plugin incompatibility. One or more plugins are not fully compatible with macOS 10.15 Catalina, yet. Did you install GPGMail.mailbundle or additional mail plugins? If so, 


Completely shut down Mail.app
Navigate to ~/Library/Mail/Bundles
Move GPGMail.mailbundle to your desktop
Restart Mail  and try again