Dispose @snapshottedView reactions when destroyed

[scott-rc]

While adding @snapshottedView to Gadget, I encountered an issue where the reaction we set up to track @snapshottedViews was still running after we detached a model from the tree:

[mobx] Encountered an uncaught exception that was thrown by a reaction or observer component, in: 'Reaction[Reaction@998]' Error: [mobx-state-tree] You are trying to read or write to an object that is no longer part of a state tree. (Object type: 'DataModel', Path upon death: '/models/DataModel-xyz', Subpath: 'type', Action: ''). Either detach nodes first, or don't use objects after removing / replacing them in the tree.

This PR disposes the reactions after the node is destroyed so we avoid this error.

[airhorns]

Ah, of course, oops! I think that just checking aliveness in the reaction works, but it means the reaction won't ever get GC'd. I think it'd be better to instead tear it down explicitly when the node is destroyed. I think we can use one of the other lifecycle hooks for that. reaction returns a disposer function that you would need to call in the destroy lifecycle hook.

[codspeed-hq]

CodSpeed Performance Report

Merging #105 will degrade performances by 48.88%

_{Comparing sc/skip-reaction-when-detached (5ba0951) with main (0ab634a)}

Summary

⚡ 1 improvements
❌ 2 regressions
✅ 8 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	sc/skip-reaction-when-detached	Change
❌	instantiating a large root	2.5 ms	4.8 ms	-48.88%
❌	accessing unmemoized getter properties of a class model	15.3 µs	19 µs	-19.56%
⚡	accessing unmemoized null property of a class model	12.2 µs	11 µs	+11.44%