AI 146250: am: CL 146238 ADT #1761064: Properly refresh UiTreeBlock when SDK is reloaded.

  The fix I did last week was just refreshing the UiRootNode
  but it turns out we need to refresh the DescriptorFilters too
  (since they are null when there's no SDK.) So the old fix was
  wrong, what we need is to generate a new tree model when the
  SDK is reloaded.
  Original author: raphael
  Merged from: //branches/cupcake/...

Automated import of CL 146250