fix(StatusAlert): Fix focus error for dismissible alert.

[MichaelRoytman]

StatusAlert requires a check for existence of xButton in componentDidUpdate to prevent errors when
displaying dismissible alerts.

[coveralls]

Coverage increased (+0.001%) to 99.506% when pulling 4ee3743 on mroytman/dismissible-status-alert-xButton into 25fe397 on master.

[coveralls]

Coverage increased (+0.001%) to 99.506% when pulling 4ee3743 on mroytman/dismissible-status-alert-xButton into 25fe397 on master.

[jaebradley]

Are there any additional tests to add so that we know if we change the component's behavior going forward?

[MichaelRoytman]

@jaebradley I think that, if I were to add testing for validation of focus-related behavior, I'd want to add test for focus logic in the StatusAlert as a whole, and I think it might be out-of-scope for me at the moment. I'm inclined to leave that until a later date, since the coverage is essentially unchanged. I'll make a note of it, though. Thank you for the review!