Combobox not bining collection inside collection on datagrid xaml WPF

ComboBox not bining collection inside collection on DataGrid xaml wpf

<pre>
xaml code inside DataGrid
<DataGridTemplateColumn Header="Unit" Width="65"> 
    <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <my:PLGDComboBox Height="23"
                             ItemsSource="{Binding Path=SaveCls.PurDtlMastCol[RowIndx].ItemUnitCol,RelativeSource={RelativeSource AncestorType={x:Type invname:MasterForm},AncestorLevel=1,Mode=FindAncestor},UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                             SelectedValue="{Binding  Path=PurUnit.UnitId,UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" 
                             DisplayMemberPath="UnitName"
                             SelectedValuePath="UnitId"/>
    </DataTemplate >
</DataGridTemplateColumn.CellTemplate>

if (dtUnit.Rows.Count > 0) 
   { 
     this.SaveCls.PurDtlMastCol[rowIndex].ItemUnitCol = new PL21000DCol(); 
     foreach (DataRow dr in dtUnit.Rows)
     {
        this.SaveCls.PurDtlMastCol[rowIndex].ItemUnitCol.Add(new PL21000D() { 
        UnitId = PLWM.Utils.CnvToInt64(dr["unit_id"]),
        UnitName = PLWM.Utils.CnvToStr(dr["unit_name"]) 
        });
     }
   }