How do I get WPF DataGrid and AutoCompleteBox's Selected Item

I've used the AutoCompleteBox on a WPF form. Now I would like to do the thing inside a WPF DataGrid. Almost everything works except the SelectedItem was not work.

In the XAML below I have added a code behind handler for LostFocus on Autocompletebox .

AutoCompleteBox autoBox = sender as AutoCompleteBox;
                if (autoBox == null)
                    return;
                
                TextBox textBox = autoBox.Template.FindName("Text", autoBox) as TextBox;
                textBox.Background = new SolidColorBrush(Color.FromArgb(0xFF, 0x33, 0x33, 0x33));
                Mouse.OverrideCursor = Cursors.Wait;
                if (txtAccount.SelectedItem != null)
                {
                    if (txtAccount.Text.Length > 0)
                    {
                        if (txtAccount.Text.Contains(" "))
                        {
                            loadAccount(0);
                            calculatePrice();
                        }
                        else
                            MessageBox.Show("Invalid account!", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
                    }
                    else
                    {
                        MessageBox.Show("Please enter text account!", "Error", MessageBoxButton.OK, MessageBoxImage.Error);
                    }
                    ddlVehicle.Focus();
                }
                Mouse.OverrideCursor = Cursors.Arrow;

here  'txtAccount.SelectedItem' is given null value ...

Please suggest how to get selecteditem on autocompletebox.