NullReferenceException on end of Play mode

In 0.12 and 0.13 on Mac OSX, getting four identical NREs.  This appears to happen when Unity3d gets focus again after Play mode.  It can happen when nothing in the scene uses a ShaderForge shader, and no ShaderForge tab is open.  Hope this is enough to reproduce, but if not, tell me what I should try here.

NullReferenceException: Object reference not set to an instance of an object

ShaderForge.SF_Node.RefreshValue (Int32 ia, Int32 ib)

ShaderForge.SF_Node_Arithmetic.RefreshValue ()

ShaderForge.SF_NodePreview.OnEnable ()