How do I solve the error "The AllSignatures.plist couldn't be opened because you don't have permission to view it"?

The first time you update signatures for Apple Mail you will get the error message below.

You get the error because the strict security model on macOS Mojave prevents the Xink app from making changes to Mac Mail.  To fix it you need to grant full disk permissions to the Xink app.

1. Go to System Preferences on your Mac.
2. Click "Security & Privacy".
3. Choose "Full Disk Access".
4. Click the "+" button (if it is disabled, click the lock in the lower left corner and enter your password).
5. Browse to the Xink.app file and click "Open".
6. Restart the Xink app.