How to retrieve data from datagrid combo box using wpf ?

Hi actually i am a newbie to the wpf and here i am facing a problem . I unable to retrieve values or data from data grid using a combo box and when ever i am trying to retrieve im getting a error "Object set to an instance" here is my code please check and help is appreciated

//XAML code :

//c# code:

DataSet ds1 = uomc.fillcombo();

cmb_pktype.ItemsSource = ds1.Tables[0].DefaultView;

cmb_pktype.DisplayMemberPath = ds1.Tables[0].Columns["Name"].ToString();

cmb_pktype.SelectedValuePath = ds1.Tables[0].Columns["Id"].ToString();

//when i am retrieveing the data like as below

string type = (dgv_uomconversionmapping.SelectedCells[1].Column.GetCellContent(dgv_uomconversionmapping.Items[i]) as TextBlock).Text;

//This retrieving process is working for data grid textblock column

//as this way how can i read the datagrid combobox column