How to conditionally show/hide toolbar buttons in Fyne?

I'm trying to let users record audio. I want a 'record' button when they aren't recording, and when they are recording to change it to a 'stop recording' button. Furthermore, I expected this to work, but both buttons always show on the toolbar:

var startRecording, stopRecordingFunc func()

recordButton := widget.NewToolbarAction(theme.MediaRecordIcon(), startRecording)
stopButton := widget.NewToolbarAction(theme.MediaStopIcon(), stopRecordingFunc)

startRecording = func() {
	if !recording {
		recording = true
		stopRecording = make(chan struct{})
		recordWg.Add(1)
		go func() {
			recordAudio("output.wav", stopRecording, &recordWg)
			recording = false
			recordButton.ToolbarObject().Show()
			stopButton.ToolbarObject().Hide()
		}()
		recordButton.ToolbarObject().Hide()
		stopButton.ToolbarObject().Show()
	}
}

stopRecordingFunc = func() {
	if recording {
		close(stopRecording)
		recordWg.Wait()
		recording = false
		recordButton.ToolbarObject().Show()
		stopButton.ToolbarObject().Hide()
	}
}

recordButton.OnActivated = startRecording
stopButton.OnActivated = stopRecordingFunc

toolbar := widget.NewToolbar(recordButton, stopButton)
stopButton.ToolbarObject().Hide()

I've tried adding calls to Refresh() but it makes no difference.

How can I conditionally show/hide toolbar buttons?

When you call .ToolbarObject() it is creating a new instance - which is not in the toolbar at all.
To manipulate the contents of a toolbar you need to use the toolbar APIs, or those exposed directly on the ToolbarAction.

From your code it looks like what you would be best using is SetIcon to change the appearance.