fix(forms): error if control is removed as a result of another one being reset

[crisbeto]

When a form is reset, it goes through _forEachChild to call reset on each of its children. The problem is that if a control is removed while the loop is running (e.g. by a subscription), the form will throw an error, because it built up the list of available controls before the loop started.

These changes fix the issue by adding a null check before invoking the callback.

Fixes #33401.

[AndrewKushnir]

Thanks for the fix @crisbeto! The original problem comes from the fact that some structural changes happen to the data structure (i.e. new controls added or old ones removed) within the valueChanges subscription fn, which is executed in the middle of the reset operation (in a sense it's similar to starting a new change detection loop while the current one is in progress). I think the current fix is good and we should definitely land it (I will take a look at the fix asap) and later on look at solving the underlying issue (for ex. by delaying the valueChanges emit and triggering it once the changes to the data structure are completed).

[AndrewKushnir]

LGTM (just one minor comment), thanks @crisbeto! 👍

[AndrewKushnir]

Presubmit.

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