React warning is thrown when leaving phone verification.

[tomalec]

Describe the bug:

React warns: Can't perform a React state update on an unmounted component when you leave the phone verification component while the verification process/promise is pending.

Expected behavior:

No warning should be thrown.

Actual behavior:

React throws:

Can't perform a React state update on an unmounted component

Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.
    in VerifyPhoneNumberContent (created by DemoVerifyPhoneNumberCard)
    in div (created by Context.Consumer)
    in CardUI (created by Card)
    in Card (created by Card)
    in Card (created by AccountCard)
    in AccountCard (created by DemoVerifyPhoneNumberCard)
    in div (created by DemoVerifyPhoneNumberCard)
    in section (created by DemoVerifyPhoneNumberCard)
    in DemoVerifyPhoneNumberCard (created by Settings)
    in div (created by Settings)
    in Settings (created by ee)
    in Suspense (created by ee)
    in ee (created by bt)
    in div (created by bt)
    in div (created by ht)
    in ht (created by bt)
    in div (created by bt)
    in slot_fill_provider_SlotFillProvider (created by Provider)
    in provider_SlotFillProvider (created by Provider)
    in Provider (created by bt)
    in bt
    in Unknown (created by WithSelect(bt))
    in WithSelect(bt) (created by WithPluginsHydration(WithSelect(bt)))
    in WithPluginsHydration(WithSelect(bt)) (created by Context.Consumer)
    in t

Warning: Each child in a list should have a unique "key" prop.

react_devtools_backend.js:4049 Warning: Each child in a list should have a unique "key" prop.

Check the render method of `ForwardRef(Button)`. It was passed a child from AppButton. See https://fb.me/react-warning-keys for more information.
    in Qc (created by AppButton)
    in ForwardRef(Button) (created by AppButton)
    in AppButton (created by VerifyPhoneNumberContent)
    in div (created by Context.Consumer)
    in Flex (created by ForwardRef(FlexComponent))
    in ForwardRef(FlexComponent) (created by VerifyPhoneNumberContent)
    in div (created by Subsection)
    in Subsection (created by VerifyPhoneNumberContent)
    in div (created by Context.Consumer)
    in BodyUI (created by CardBody)
    in CardBody (created by Body)
    in Body (created by VerifyPhoneNumberContent)
    in VerifyPhoneNumberContent (created by DemoVerifyPhoneNumberCard)
    in div (created by Context.Consumer)
    in CardUI (created by Card)
    in Card (created by Card)
    in Card (created by AccountCard)
    in AccountCard (created by DemoVerifyPhoneNumberCard)
    in div (created by DemoVerifyPhoneNumberCard)
    in section (created by DemoVerifyPhoneNumberCard)
    in DemoVerifyPhoneNumberCard (created by Settings)
    in div (created by Settings)
    in Settings (created by ee)
    in Suspense (created by ee)
    in ee (created by bt)
    in div (created by bt)
    in div (created by ht)
    in ht (created by bt)
    in div (created by bt)
    in slot_fill_provider_SlotFillProvider (created by Provider)
    in provider_SlotFillProvider (created by Provider)
    in Provider (created by bt)
    in bt
    in Unknown (created by WithSelect(bt))
    in WithSelect(bt) (created by WithPluginsHydration(WithSelect(bt)))
    in WithPluginsHydration(WithSelect(bt)) (created by Context.Consumer)
    in t
    in t
    in t
    in Unknown (created by WithOptionsHydration(Component))
    in WithOptionsHydration(Component) (created by WithSettingsHydration(WithOptionsHydration(Component)))
    in WithSettingsHydration(WithOptionsHydration(Component)) (created by WithSettingsHydration(WithSettingsHydration(WithOptionsHydration(Component))))
    in WithSettingsHydration(WithSettingsHydration(WithOptionsHydration(Component))) (created by WithCurrentUserHydration(WithSettingsHydration(WithSettingsHydration(WithOptionsHydration(Component)))))
    in WithCurrentUserHydration(WithSettingsHydration(WithSettingsHydration(WithOptionsHydration(Component))))

Steps to reproduce:

1. Go to phone verification settings /wp-admin/admin.php?page=wc-admin&path=%2Fgoogle%2Fsettings
2. Click "Send verification code"
3. Input 123456
4. Navigate away

Additional details:

• I guess Each child in a list should have a unique "key" prop is a separate issue for the Button itself, but was not sure.
• Seems to be related to my doubt about promise handling in Implement new components for phone verification process #993 (comment)