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If you change the tests to
Does that fix it?
if ('browserEvent' in <whateverObjectYouAreLookingAt>)
Can't recreate the condition at the moment, possibly due to my workaround.
I tried undoing it and recreating, but no luck as yet.
I'd assume your fix is fine, and certainly faster than calling isDefined.
Saying that though, I'd assume event.browserEvent to be truthy, so that's assumptions for ya