An unexpected error occurred. Call to a member function label() on null

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I just noticed a problem with my website when editing custom page "Attributes". I get An unexpected error occurred. Call to a member function label() on null. This must have occurred during one of the upgrades to C5 but it's too late to revert back.

Any help out there would be appreciated.

Also, I noticed I have a duplicate "Thumbnail" attribute when editing a pages attributes. This is an attribute I added at one point. The other common page attributes seem to work just fine.

haundavid