VisualBrush triggers a NRE when used in a OpacityMask.

[jmacato]

To repro: Just set any visual control's OpacityMask with a VisualBrush. On runtime it should trigger an NRE.

[jmacato]

Repro:

    <Rectangle Width="300" Height="300" Fill="Red">
      <Rectangle.OpacityMask>
        <VisualBrush>
          <VisualBrush.Visual>
            <Rectangle Height="100" Width="100" Fill="Black" />
          </VisualBrush.Visual>
          
        </VisualBrush>
      </Rectangle.OpacityMask>
    </Rectangle>

@grokys upon checking, seems like that the VisualBrush never do a Measure() call on VisualBrush's Visual if its defined in XAML, hence the null exceptions...