how to delete rows contains primary key null value in datagrid view

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C#

C#4.0

DataGridView

Projects.cmdb = new OleDbCommandBuilder(Projects.da);
                    Projects.ds.GetChanges();
                    DataRow rowFound;
                    DataTable tb = Projects.ds.Tables[0];
                    DataColumn[] dc = new DataColumn[1];
                    dc[0] = tb.Columns["UNITSLNO"];
                   
                    tb.PrimaryKey = dc;
                    tb.DefaultView.Sort = "UNITSLNO";
                  
                        if (tb.Rows.Contains(""))
                        {
                       
                            rowFound = tb.Rows.Find("");
                            Projects.ds.Tables[0].Rows.Remove(rowFound);
                            Projects.ds.AcceptChanges();
                        }

the above code as it seems should delete all null rows (any way primary column will not have null value!!!!)if any ,in the datagridview . this i did because if the end user try to add a null value in the primary index key column "UNITSLNO",by mistake ,it should be deleted by executing the above code without further error handling. but unfortunately it do not remove the null rows. i tried with some valid numbers; while executing this code the row got deleted. why it is not happening with null;