I have a DataGrid I want to edit. One column is a Combobox 

```
<DataGrid ItemsSource="{Binding Persons, Mode=TwoWay}"
          SelectedItem="{Binding SelectedPerson, Mode=TwoWay}" >
     <DataGrid.Columns>
         <DataGridTemplateColumn Header="Company" >
            <DataGridTemplateColumn.CellEditingTemplate>
                <DataTemplate>
                    <ComboBox ItemsSource="{Binding RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}, Path=DataContext.Companies}"/>
                </DataTemplate>
            </DataGridTemplateColumn.CellEditingTemplate>
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Company.Name}" />
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>
    </DataGrid.Columns>
</DataGrid>

```

The problem is if the Company is `null` then I get the error

> 
> Two-way binding requires Path or XPath
> 
> 
> 

How can I get around that to allow the Company be set to null?