I hope I'm creating this ticket in the correct section.
If not, please feel free to move it.
The MockAsset.getRenditions method currently returns an ImmutableList:
This does not seem like a big deal, but when calling UIHelper.getBestfitRendition we end up in DamUtil.getBestFitRendition, which tries to sort the renditions when no perfect match can be found. Hence, an UnsupportedOperationException is thrown.