Okay i got a moment to take a look at line 913
which PreviousField is
private string PreviousField
return ValidationHelper.GetString(ViewState["PreviousValue"], string.Empty);
ViewState["PreviousValue"] = value;
So you aren't crazy, it looks like it leverages the previous field in it's loading of the control setting. This code hasn't really changed from V10 to V11, so i'm thinking that something is not setting this PreviousField in the viewstate properly. The error makes sense now, but my guess is Support will need to look into why selecting a Category header screws this up (probably sets a null to the PreviousValue or something).