Hi Shane - Thanks for the response. I just commented out the line of code in Task.cs :
// CompletedOnDate = Null.SetNullDateTime(dr["CompletedOnDate"]);
I was still getting an error on the modal popup. I ended up using NavigateURL instead of EditUrl in the Response.Redirect call in View.ascx.cs. I noticed some blogs mentioned that EditUrl doesn't work well in DNN6+