Null messages

If you’re using mail, and you’re using POP to read and retrieve your mail, and you have multiple mail clients sharing the mail account, this is normal and expected behavior. With POP, the first mail client to the mail message wins, and downloads the message to the device. Switch to IMAP, if that’s available with your mail provider.

Email or iMessages ??

Null usually means "no signal" but don't know in your case as you do not say if you are only getting the notification...and still getting the message?...or not getting the message?...or if it is happening to just one contact?...happening all the time?...or just sometimes?...etc.

If because of signal try a place where signal is better to see if they stop...or have the contact sending the message try another place.