title: Property can not be null on Trigger
body: <pre><code>&lt;Style TargetType="controls:EventTimeView"&gt;      
      &lt;Style.Triggers&gt;         
          &lt;Trigger Property="PositioningMethod" Value="Column" &gt;
              &lt;Setter Property="Background" Value="Black" /&gt;
          &lt;/Trigger&gt;
      &lt;/Style.Triggers&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p>PositioningMethod is a dependency property of EventTimeView of an enum type.</p>
  
  <p>Results in:</p>
  
  <pre><code>System.InvalidOperationException occurred
  Message=Property can not be null on Trigger.
  Source=PresentationFramework
  StackTrace:
     at System.Windows.StyleHelper.UpdateTables(PropertyValue&amp; propertyValue, FrugalStructList`1&amp; childRecordFromChildIndex, FrugalStructList`1&amp; triggerSourceRecordFromChildIndex, FrugalStructList`1&amp; resourceDependents, HybridDictionary&amp; dataTriggerRecordFromBinding, HybridDictionary childIndexFromChildName, Boolean&amp; hasInstanceValues)
  InnerException: 
  </code></pre>
  
  <p>I have no idea what might be wrong. The exception is too vague: which property? what is null?</p>