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Null Reference Exception on Recognition of Second Target

Hi.

I can scan any of my targets, but when I close the trackable, and try to scan another one, the scan lines disappear and I get the below error:

NullReferenceException
  at StateManagerImpl.RemoveDisabledTrackablesFromQueue (System.Collections.Generic.LinkedList`1& trackableIDs) [0x00000] in <filename unknown>:0 
  at QCARManagerImpl.UpdateTrackers (FrameState frameState) [0x00000] in <filename unknown>:0 
  at QCARManagerImpl.Update (ScreenOrientation counterRotation, CameraDeviceMode deviceMode, .VideoModeData& videoMode) [0x00000] in <filename unknown>:0 
  at QCARBehaviour.Update () [0x00000] in <filename unknown>:0 

It's not isolated to any particular trackable, but it is always the second trackable i.e. once a trackable has already appeared, the next one crashes.

Can anybody help with this?