Refactor Anchored Region stories to remove non-null-assertions

[chrisdholt]

As part of #5303 we added configuration to disallow non-null-assertions. Anchored region stories within fast-components includes several non-null-assertions which should be fixed-up. This task exists to track the removal of the flag disabling that check and fix up the necessary code to remove the warnings.

[m4thieulavoie]

@chrisdholt I would be happy to take care of that if you want to assign this issue to me 👍

[chrisdholt]

@chrisdholt I would be happy to take care of that if you want to assign this issue to me 👍

Done!