This does not look as the same error that Joe is getting. From the url on his screenshot and his description it looks like that he is going to Permissions section directly and he is not going to edit the backend page.
Anyway, I am currently using the latest internal build and I am not able to see this issue. Testing with the beta also does not throw object reference for me.
If you want you can open Support ticket and attach your project for review, but I cannot reproduce the issue with the BETA or latest version from source control.
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