That's a .net error, which, looking at your code I don't understand, as you've specified 3 parameters and also the indexes are specified properly. If you run that code in a small console app, it should give the same error.
The thing is that the grid at DESIGN TIME works with the properties on an entity as if it's runtime, i.e. reads the values of them. If something goes wrong while doing that, the exception shows up in the grid/bound control in the DESIGNER. So it might look like a designer issue, it is an execption originating in your code.