title: Set null to property in mvvm WPF when nothing is filled in textbox
body: <p>I implemented a textbox, and this binds to a viewmodel. I fill in the textbox default by '100' for example, but if i change this to 10, the property always sets correctly. But when i delete all the numbers, i want that the property is set to null. But it just don't set the value when nothing is filled in. He just keeps the last value..</p>
  
  <p>this is my code , viewmodel + xaml:</p>
  
  <pre><code>public double MaxTime
  {
      get
      {
          return maxTime;
      }
      set
      {
          maxTime = value;
          OnPropertyChanged("MaxTime");
          if (SelectedQuestionDropList != null)
          {
              foreach (QuestionCluster cluster in this.Examination.QuestionClusters)
              {
                  if (cluster == SelectedQuestionDropList)
                  {
                      cluster.MaxTime = value;
                  }
              }
          }
      }
  }
  
  
  &lt;TextBox Height="23" 
           Visibility="{Binding Path=VisibleClusterDetails, Converter={StaticResource boolToVis},UpdateSourceTrigger=PropertyChanged}"  
           Text="{Binding Path=MaxTime,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}"
           HorizontalAlignment="Right" Margin="0,511,601,0" Name="textBox2" 
           VerticalAlignment="Top" Width="120" /&gt; 
  </code></pre>