Add guard for destroyed modal in handleTabIntoBrowser

This PR adds a guard to handleTabIntoBrowser to do an early return if
the modal is in a destroyed state.

During our integration tests, our teardown hook was destroying the app
instance but we weren't explicitly closing the modal. When we were
hitting our subsequent tests, ic-modal was trying to focus in on the
modal even though it was in a destroyed state. While we're now making
sure we explicitly close the modal in our teardown hook, it feels like
ic-modal should internally handle this and guard against trying to focus
on a destroyed modal.