title: Combobox SelectedItem doesn't show
body: <p>I have the following <code>Combobox</code>:  </p>
  
  <pre><code>&lt;ComboBox Name="cb_Zeitraum" Grid.Column="2" Grid.Row="0" ItemsSource="{Binding Zeitraum}" IsEnabled="{Binding TimespanAvailable}" SelectedItem="{Binding Zeit}" SelectedIndex="0" Style="{StaticResource requiredFieldValidationStyle}" HorizontalAlignment="Left" Width="26"/&gt;  
  </code></pre>
  
  <p>My Problem is when I select an <code>Item</code> it isn't displayed inside the <code>Combobox</code>, it just stays blank. But the the <code>Item</code> is selected, just not shown. It just stopped working. I have reversed all the changes I made after it stopped working but that didn't help.  </p>
  
  <pre><code>&lt;!-- Required Fields --&gt;
  &lt;Style x:Key="requiredFieldValidationStyle"&gt;
      &lt;Style.Triggers&gt;
          &lt;DataTrigger Binding="{Binding RelativeSource={RelativeSource Self}, Path=Text}" Value=""&gt;
              &lt;Setter Property="TextBox.BorderBrush" Value="Red"/&gt;
              &lt;Setter Property="TextBox.ToolTip" Value="Eine Eingabe wird benötigt."/&gt;
          &lt;/DataTrigger&gt;
      &lt;/Style.Triggers&gt;
  &lt;/Style&gt;  
  </code></pre>
  
  <p><strong>Picture:</strong><br>
  <img src="https://i.stack.imgur.com/gaZg3.png" alt="Combobox Problem"></p>
  
  <p><strong>Declaration:</strong>  </p>
  
  <pre><code>private ObservableCollection&lt;string&gt; _zeitraum = new ObservableCollection&lt;string&gt;();
      public ObservableCollection&lt;string&gt; Zeitraum
      {
          get { return _zeitraum; }
          set { _zeitraum = value; }
      }
  
  private string _zeit;
      public string Zeit
      {
          get { return _zeit; }
          set
          {
              _zeit = value;
              NotifyPropertyChanged("Zeit");
          }
      }
  </code></pre>