[Solved] Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints

I am facing this error in binding of DropDownList in Grid.

I am using TableAdapter inside DataSet.xsd (DLL)

.aspx.cs code to bind DropDownList inside Grid:

GridEditableItem item = e.Item as GridEditableItem;

            DropDownList rlist = item.FindControl("ddlRelationship") as DropDownList;
            rlist.DataTextField = "RName";              
            rlist.DataValueField = "RID";
            rlist.DataSource = SDM.BU.GetAllRelationship();
            rlist.DataBind();

            foreach (DataRow dr in _dtBU.Rows)
            {
                if (dr["ID"].ToString() == item.GetDataKeyValue("ID").ToString())
                {                        
                    rlist.SelectedValue = dr["BURelationship"].ToString();    
                    break;
                }
            }

SDM.BU class file code (BLL):

#region GET
    public SDMDAL.SDM_Master_BUDataTable GetBU()
    {
        return Adapter.GetBU();
    }

    public DataTable GetAllRelationship()
    {
        DataTable dt = Adapter.GetRelationship();
        return dt;
    }        
    #endregion

Stored Procedure to bind the DropDownList is 

ALTER PROCEDURE [dbo].[SDM_Select_Relationship]
    AS
BEGIN
    SET NOCOUNT ON;

    SELECT distinct [RID], [RName] 
    FROM [SDM_DB].[dbo].[SDM_Master_Relationship](NOLOCK)
    ORDER BY [RID] 
END

Database Table which I am using to bind the DropDownList:

http://i.imgur.com/DVDuPmh.png

here, RfoStatusID is the foreign Key.

DataSet TableAdapter

http://i.stack.imgur.com/giew1.png

Please let me know the cause of this error and how to resolve it ? I am unable to understand why this error is coming. Whenever I click on Add New button of RadGrid, Add panel opens with this Logging Exception. Please let me know What is wrong in my code ?  NOTE: I am very new in Table Adapter method and 1st time using it.  Please reply