I know this error, it is annoying 😛
Unfortunately, we have no control over this but we are trying to get some clarification from Microsoft regarding this issue.
The error only appears in Design view. You can still run your application without problem.
If you remove LayerIDs in your Legend, this error will go away.
The only thing I can say is that our team is aware of this Design view error and we too are finding answers about how we can possibly get this issue resolved or atleast workaround so that our users can still see Design view without the annoying error popping up 😛
However, when the same legend control is incorperated/merged into an app based on either the standard or the showcase template, that legend gets stuck. I am talking about 3.0 API here. How could one explain that it works with one ESRI thing and doesn't with another ESRI thing?
You pointed out a design issue that should not impact the runtime.
I don't figure out any reason for that. Could you share more détails on how to reproduce this issue?
Thanks
Look at this incident which is more than two months old.
Esri Incident #1113122.
The we all including the analyst got very frustrated.
I have map services that I used in creating a simple app using the SL Viewer GUI and I would like to use the same map services (some services have sub-layers) for an re-invented app with additional functionality and got stuck with this issue. If you can't find the notes for the incident, just drop a note here and I can supply them. Thanks in advance for looking into this.