Cannot switch back from nengo-fpga backend in the GUI

[bmorcos]

Working on a demo I added nengo-fpga as a backend to the GUI (in nengo_gui/exec_env.py).

I can switch between nengo and nengo-ocl backends freely, and from either of these to nengo-fpga. However, I cannot switch back from nengo-fpga to either nengo or nengo-ocl.

I haven't looked into this since it doesn't seem like a frequent use case, but my random guess:
I use the FpgaPesEnsembleNetwork with the other backends, and this simple uses the dummy ensemble inside the FPGA network object. Once I switch to the nengo-fpga backend, the FPGA is built and properly connected, but switching back to nengo doesn't disconnect or cause the model to rebuild the FPGA network with the dummy ensemble, so it hangs.