This happened after the OnDemand fixes and addition of barriers.

When setting the framegraph surface to nullptr and then resetting it to a valid surface, nothing happens. The render thread is hanging in Renderer::waitForRenderJobs(), so I assume it is a deadlock. 

This is easy to reproduce. I modified the basicshapes-cpp example by adding two buttons, one that sets the surface in the framegraph surface selector to null, the other to the original surface.