Hello, @johnsaxton
In this particular case, the field "Email Addresses" is empty that is why you get the error. Please enter the E-mails of those users which you want to invite. It is not necessary they belong to your or Microsoft domain - it can be any mail service (Gmail, Yahoo, iCloud and so on).
I tried to reproduce the issue passing to the action null in different ways but didn't succeed - there is always an empty string. Could you show your flow with opened properties so I can find out how the null value got in the field?