Fix a few bugs related to custom events

[farskid]

Fixes https://github.com/statelyai/xstate-viz/issues/250 https://github.com/statelyai/xstate-viz/issues/222 https://github.com/statelyai/xstate-viz/issues/223

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[linear[bot]]

STA-578 Send button is disabled in the custom event editor

[farskid]

Could we also add some basic tests for this thing?

It would also be nice if we could explain such changes, at least briefly. What has been previously assumed? What has happened? How it got fixed? And why the new approach is better? I had to figure this stuff out on my own when reviewing this - and having some kind of an explanation/reasoning behind the changes would help me a lot.

I'm on it.

[davidkpiano]

@farskid @Andarist Besides tests, what are the remaining changes here?