Panic during network activation update

[rohrlich]

Clicking on a different tab of simulation GUI has sometimes caused a panic in updating the UI but this is the first time I've seen this stack trace.

runtime.fatalpanic at panic.go:847
runtime.gopanic at panic.go:722
sync.(*WaitGroup).Wait at waitgroup.go:132
github.com/emer/leabra/leabra.(*NetworkStru).ThrLayFun at networkstru.go:622
github.com/emer/leabra/leabra.(*Network).SendGDelta at network.go:168
github.com/emer/leabra/leabra.(*Network).Cycle at network.go:157
github.com/emer/leabra/deep.(*Network).Cycle at network.go:126
main.(*Sim).AlphaCyc at audonly.go:575
main.(*Sim).TrainTrial at audonly.go:701
main.(*Sim).Train at audonly.go:1024
runtime.goexit at asm_amd64.s:1357

• Async stack trace
  main.(*Sim).ConfigGui.func2 at audonly.go:1813