Sure. It's basically caused by a bug in our First Device add experience. If you click the tile for 'Bring your own Thing' or 'Arduino' before going back and selecting the tile for Raspberry Pi, it will cause a "null" invite code to be generated for the Pi.
The fix is to remove the pending Arduino or BYOT device from your account. But you can't access the device list to do this until you have a device on your account! To get another device on your account, you have a few options:
1) Add an actual Arduino, BYOT, or LoRa device to your account, so you can access the device list, remove the 'Pending' entries on your account, and then add your Raspberry Pi (a non-null code would be generated at this point). The pending entries show up grey with an hourglass icon in the sidebar, like this:
2) If you don't have any other device handy to add besides your Raspberry Pi, you can either add your Pi as a BYOT device following these instructions, or generate a 'fake' MQTT device by entering your credentials into MQTT software like MQTT.fx, as described in this documentation.
I just offered to help since a lot of people who've encountered this have no other device, and it's easy for me to add one to their account, fix the issue, and delete it. Certainly if you have questions about the above let me know, and if anyone else is reading and wants me to fix this issue for your account, just reply here and @mention me, or send me a private message.