Manager API: Fix api.getAddonStatedefault value

[sookmax]

Closes #23741 #23787

What I did

I've noticed api.getAddonState() sometimes (not sure the exact condition) returns undefined, which affects consumers of api.setAddonState().

More specifically, #23787 occurs when the state updater function setCount() in code/addon/actions/src/manager.tsx attempts to access c.count assuming the state { count: number; } is available all the time (i.e., setCount((c) => ({ ...c, count: c.count + 1 }));). Unfortunately as I mentioned above, sometimes api.getAddonState(), which is the value of the parameter c of the callback in setCount(), results in undefined.

This PR fixes the issue by providing defaultState when c is undefined in useSharedState().

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "build", "documentation", "maintenance", "dependencies", "other"]

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[ndelangen]

The confusion comes from a race condition:

When using useAddonState() in 2 places in the react tree, or even across manager & preview... then if both do not have the exact same default config there's the possibility of creating an infinite loop.

So it's recommended to only have the default state value defined in 1 place. everywhere else you might get an undefined initially, but it will get called again.

[sookmax]

Oh I see.. so it's more involved than I originally thought.

But my fix merely uses the default state provided to useSharedState, which was already there before the fix, and even the initial undefined passed here: setCount((c) => ({ ...c, count: c.count + 1 })) will produce a runtime error and the counter becomes out of sync with the number of items shown in the action panel.

[kasperpeulen]

Comitted on this one, so I can not approve anymore 😅