Crash in Switches.OnChange: SelectedItem() or .Value can be nil in a Mutex mode, but it should never be.

[rcoreilly]

Describe the bug

emergent/egui/loopctrl.go:72 is the crash:

		tree.AddAt(p, pfx+"loop-mode", func(w *core.Switches) {
			w.SetType(core.SwitchSegmentedButton)
			w.Mutex = true
			w.SetEnums(modes...)
			w.SelectValue(curMode)
			w.FinalStyler(func(s *styles.Style) {
				s.Grow.Set(0, 0)
			})
			w.OnChange(func(e events.Event) {
-> line 72:		curMode = w.SelectedItem().Value.(enums.Enum)
				st := loops.Stacks[curMode]
				if st != nil {
					curStep = st.StepLevel
				}
				updateSteps()
				stepChoose.Update()
				stepN := st.Loops[curStep].StepCount
				stepNSpin.SetValue(float32(stepN))
				stepNSpin.Update()
			})
		})

How to reproduce

run ccn sims on web, probably in a slow system? I didn't get this on my mac.

Example code

No response

Relevant output

No response

Platform

macOS

[kkoreilly]

Need a reproducible test case before I can fix this.