"null" with my email

Hello,

Try to sign out of iMessage by pulling down the Messages menu -> Preferences -> Sign out. Restart your Mac by pulling down the Apple Menu  -> Restart. Once your Macintosh has started back up open Messages and sign back into iMessage. Also verify that Messages for iCloud is enabled in System Preferences -> iCloud.

I think it has to do with this...

Ah, but here's the rub: You can only use iMessage with other iDevice owners. If you have an iPhone and you use the Messages app to contact an Android, BlackBerry or Windows Phone user, iOS recognizes there's no iMessage at the other end and switches (downshifts?) into SMS mode. Your indication that has happened? Green word bubbles instead of blue.

However, you may end up seeing green even if you're corresponding with another iPhone. There are three possible causes:

https://www.cnet.com/how-to/why-some-imessage-texts-are-blue-and-some-are-green/