I have two RadioButtons

```
 private void CarRadioButton_Checked(object sender, RoutedEventArgs e)
        {
            try
            {

                // When CarRadioButton is clicked. "Leasing" and "Sale" is added 
                   to Combobox

                ContractComboBox.Items.Clear();

                ContractComboBox.Items.Add("Leasing");
                ContractComboBox.Items.Add("Sale");

           }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

        }

        private void TruckRadioButton_Checked(object sender, RoutedEventArgs e)
        {
           try
            {
                 // When TruckRadioButton is clicked. "Leasing" is added 
                   to Combobox

                ContractComboBox.Items.Clear();

                ContractComboBox.Items.Add("Leasing");

            }
            catch (Exception ex)
            {
             MessageBox.Show(ex.Message);
            }

        }

```

When I select an item from the `ComboBox`, I want to do something, like the following:

```
  private void ContractComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (ContractComboBox.SelectedItem.ToString() == "Leasing")
            {
                PriceMonthTextBox.IsEnabled = true;
                PeriodTextBox.IsEnabled = true;
            }

            if (ContractComboBox.SelectedItem.ToString() == "Sale")
            {
                PriceMonthTextBox.IsEnabled = false;
                PeriodTextBox.IsEnabled = false;
            }

        }    

```

Now here is my problem. If I select an item from the `ComboBox` and after that click on another `RadioButton` I get an error: `Object reference not set to an instance of an object.`

Steps I did leading to the exception:

1. `CarRadioButton` Clicked.
2. Picked "Leasing" from `ComboBox`
3. I click on `TruckRadioButton`
4. Get error: `Object reference not set to an instance of an object`.

Any ideas? i think its a little problem but i cant spot it :(