title: DataGrid looses focus but SelectedItem is not set to null
body: <p>I have a DataGrid in WPF. I have binded SelectedItem property. I have Remove button which removes selected row. However when I click to other place (i.e. TextBox) and DataGrid has lost a focus, SelectedItem does not change. I can Remove the row, that was selected "earlier". </p>
  
  <p>Is it default behaviour you would expected? Can it be easily fixed?</p>
  
  <p>edit (code as requested):</p>
  
  <pre><code>&lt;DataGrid SelectedItem="{Binding SelectedPort}" (...) &gt;
  
  &lt;Button Command="{Binding RemovePortCommand}" &gt;Remove Port&lt;/Button&gt;
  
  public ICommand RemovePortCommand
  {
      get
      {
          return _removePortCommand ?? (_removePortCommand = new RelayCommand(param =&gt; OnRemovePortCommand(), param =&gt; SelectedPort != null));
      }
  }
  </code></pre>