title: items collection must be empty before using itemssource - datagrid wpf
body: <p>I keep getting this error when trying to explicitely define the datagrid columns.  If I comment out the 3 datagridtext columns, it works (but I want to hide some columns obviously).      In my code, all I am doing is setting the datacontext for DataGridLookupsTab..not messing with itemsource or anything in my code.  Any idea what I might be doing wrong here ? </p>
  
  <p>Thanks ! </p>
  
  <pre><code>&lt;TabItem Header="Lookups" Height="23" VerticalAlignment="Top"&gt;
              &lt;Grid x:Name="DataGridLookupsTab" Background="#FFE5E5E5" Margin="0,0,-1,-2" DataContext="Apps.OMS.Models.Lookups"&gt;
                  &lt;ComboBox x:Name="ComboBoxDistinctLookups" HorizontalAlignment="Left" Margin="128,14,0,0" VerticalAlignment="Top" Width="228" ItemsSource="{Binding Path=LookupTypeList}" SelectionChanged="ComboBoxDistinctLookups_SelectionChanged"/&gt;
                  &lt;Label Content="Select Lookup Type" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" RenderTransformOrigin="1.105,4.731"/&gt;
                  &lt;DataGrid x:Name="DataGridLookups" HorizontalAlignment="Left" Margin="10,41,0,0" VerticalAlignment="Top" Width="768" Height="327" ItemsSource="{Binding LookupRecords}" &gt; 
                      &lt;DataGridTextColumn Binding="{Binding LookupType}" Header="Lookup Type" /&gt;
                      &lt;DataGridTextColumn Binding="{Binding LookupValue}" Header="Lookup Value" /&gt;
                      &lt;DataGridTextColumn Binding="{Binding LookupDescription}" Header="Lookup Description" /&gt;
                  &lt;/DataGrid&gt;
              &lt;/Grid&gt;
          &lt;/TabItem&gt;
  </code></pre>