If you see Apple Mail Preferences as a blank box with no options, try to follow these steps and the Preferences should start showing fine and allow you to manage your Signatures section in Apple Mail.
- Quit Mail app. Make sure you quit it (click "Activity Monitor" and check if Apple Mail is not working).
- Go to Finder and click Go>Go to Folder.
- Rename the ~/Library/Mail folder.
- Delete ~/Library/Containers/com.apple.mail.
- Open Mail app.
Then Apple Mail should download all IMAP folders.
When you check that all is ok, delete the renamed folder in ~/Library.