No mail preferences in High Sierra

Just updated to High Sierra. Now selecting Mail Preferences results in a blank grey box. Cool...but not at all.

I've looked through a lot of forum posts:

• I don't have a containers folder
• I don't have any of these items
  • ~/Library/Application Support/CloudDocs/session/containers/com.apple.mail.plist
  • ~/Library/Library/Preferences/com.apple.mail.plist
  • ~/Library/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.m ail.plist
  • ~/Library/Library/Containers/com.apple.mail/Data/Library/SyncedPreferences/com.a pple.mail.plist

My signatures are now also missing. I was advised to turn on iCloud for mail by the post in the screen shot below. Instead, it removed my signatures from the drop down menu completely (they used to be in the drop down menu, but I couldn't access editing them because preferences wasn't available).