Invalid Markup in xaml design view - except app runs - how to fix Invalid Markup message (VS2012)?

I have a test wpf app I created in VS2012.  It is based on a sample from VS2008.  It is a datagrid wpf sample that displays data from the Customer table inNorthwind db.  I am using the wpf2008toolkit dll for the datagrid.  The app opens up the datagrid display window from a xaml Menu window (the main window).  This is running OK, but in the design view of the datagrid display window  (called "DatasetCRUDExample.xaml")  I have this "Invalid Markup" message in place where the datarid should be.  Note:  I added a subfolder to the project called"DatasetExamples" which is the way the 2008 sample project had it.  The"CustomerDataProvider" class resides in a class file called"NorthwindDataProvider.cs" (not using MVVM) that resides in the"DatasetExamples"  subfolder.   The actual error message is this:

Error 1 Nested types are not supported

 I would appreciate a suggestion for a workaround for this error.  Actually, in VS2012 -- do I even need to use wpf2008tookkit dll for the datagrid?  How could I do this without using wpf2008toolkit?  Below is a pictures of my VS2012 design view problem and below that is the code behind for the "NorthwindDataProvider.cs"

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using myWPFdatagridTestApp.NorthwindDatasetTableAdapters;

namespace myWPFdatagridTestApp.DatasetExamples
{
    public class NorthWindDataProvider
    {
        private static NorthwindDataset northwindDataSet;

        public static NorthwindDataset NorthwindDataSet
        {
            get
            {
                if (northwindDataSet == null)
                {
                    northwindDataSet = new NorthwindDataset();
                }
                return NorthWindDataProvider.northwindDataSet;
            }
        }    
    }

    public class CustomerDataProvider    {
        private CustomersTableAdapter adapter;

        public CustomerDataProvider()
        {
            NorthwindDataset dataset = NorthWindDataProvider.NorthwindDataSet;
            NorthWindDataProvider.NorthwindDataSet.Clear();
            NorthWindDataProvider.NorthwindDataSet.Customers.Clear();
            adapter = new CustomersTableAdapter();
            adapter.Fill(NorthWindDataProvider.NorthwindDataSet.Customers);            
        }

        void CustomersRowModified(object sender, NorthwindDataset.CustomersRowChangeEvent e)
        {
            adapter.Update(NorthWindDataProvider.NorthwindDataSet.Customers);
        }

        public DataView GetCustomers()
        {
            return NorthWindDataProvider.NorthwindDataSet.Customers.DefaultView;
        }
    }
}

Rich P