Blast from the past! We're still waiting for a bug fix for this issue to be approved for our production systems but we have found a workaround for the 'null' in the URL. Even if you're not interested I want to post it here for anyone who runs into this issue and finds this thread so they can have a resolution.
The problem is that you have a "Pending" Raspberry Pi device on your account right now with a null code. If we can delete it and then re-create it, it should generate a code. Complicating things is that our 'first device' experience when you have no other devices on your account won't allow you to reach the device list to do this on your own.
The solution is to add another non-Raspberry Pi device to your account temporarily so you can access the device list, delete the problem Pi entry, then re-create it. So how do you do this? There are a few options:
The easy solution: Send me a private message with your account login and password by clicking on my name on this post, then the blue 'Message' button. Then I'll take care of fixing this and hand the account back off to you. If you'd like, you can change your password before and after sharing the details at this link.
If you don't want to share your account login, I understand. If you have an Arduino device, add it to your account by logging in to the web dashboard and choosing the Arduino button when prompted to add a device. Once you've added it, you should have access to the device list on the left side, where you can delete the 'pending' Raspberry Pi entry by clicking it, then the 'Remove' button. Then you can re-add a new Raspberry Pi via Add New > Device/Widget > Single Board Computers > Raspberry Pi, and finally delete the temporary Arduino.
If you don't want to share your login AND you don't have an Arduino, Go through the Add Arduino device process anyway until you get to the screen where it offers an 'Auth Token'. Then message me with just that token and I'll add an Arduino to your account so you can complete the rest of option 2 above.
Thanks for your patience while we work on this, and let me know if you have any questions or concerns about the above.