Adding a guard for null activeHover | WEB-4080

@jnu for review

This is a bit blind, in that all I know is that we got a stacktrace from production pointing to a particular line and column with the error `TypeError: Cannot read property 'data' of null`. But given that the line was in `closeDesyncedTip` and seemed to clearly indicate that `activeHover` was null here, and given that as far as I can tell the correct action in this case is nothing, I think simply adding the extra guard here is a reasonable precaution.