C# comboBox_SelectionChanged

I want to display those values in the textbox when the comboBox_SelectionChanged is activated:

  private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            int selectedIndex = comboBox1.SelectedIndex;

            textBox1.Text = "";
            string[] hr = { "24", "8.8", "8.5", "11.55", "11.5", "17.3", "20.3", "20.05", "23.05" };
            textBox1.Text = hr[selectedIndex];

            MessageBox.Show(hr[selectedIndex]);
        }

Those values can be displayed in the MessageBox when the SelectionChanged is activated.

But why the code does not pass through for the textBox?

it keeps on complain about System.NullReferenceException

System.NullReferenceException was unhandled by user code
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=TestSystemUtilisation
  StackTrace:
       at TestSystemUtilisation.MainWindow.comboBox1_SelectionChanged(Object sender, SelectionChangedEventArgs e) in C:\DriveD\TESTS4WORK\TestSystemUtilisation\MainWindow.xaml.cs:line 157
       at System.Windows.Controls.SelectionChangedEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
       at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
       at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
       at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
       at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
       at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
       at System.Windows.Controls.ComboBox.OnSelectionChanged(SelectionChangedEventArgs e)
       at System.Windows.Controls.Primitives.Selector.InvokeSelectionChanged(List`1 unselectedInfos, List`1 selectedInfos)
       at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
       at System.Windows.Controls.Primitives.Selector.SelectionChanger.SelectJustThisItem(ItemInfo info, Boolean assumeInItemsCollection)
       at System.Windows.Controls.Primitives.Selector.OnSelectedIndexChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
       at System.Windows.DependencyObject.CoerceValue(DependencyProperty dp)
       at System.Windows.Controls.Primitives.Selector.OnItemsChanged(NotifyCollectionChangedEventArgs e)
       at System.Windows.Controls.ItemsControl.OnItemCollectionChanged2(Object sender, NotifyCollectionChangedEventArgs e)
       at System.Collections.Specialized.NotifyCollectionChangedEventHandler.Invoke(Object sender, NotifyCollectionChangedEventArgs e)
       at System.Windows.Data.CollectionView.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
       at System.Windows.Controls.ItemCollection.RefreshOverride()
       at System.Windows.Data.CollectionView.RefreshInternal()
       at System.Windows.Data.CollectionView.Refresh()
       at System.Windows.Controls.ItemCollection.EndInit()
       at System.Windows.Controls.ItemsControl.EndInit()
       at MS.Internal.Xaml.Runtime.ClrObjectRuntime.InitializationGuard(XamlType xamlType, Object obj, Boolean begin)
  InnerException: