BL-729: Timer.tick error - Cannot access a disposed object

I'm not clear why we need both these fixes. If we are disposed, the timer is stopped and disposed of, so it can't tick. Unless we can be disposed on some other thread (in which case we'd probably need to invoke disposing of the timer), I don't see how this fix can be needed.

Unless of course we got a new DownloadedBooksChanged event and started a new timer after this was disposed...but the fix for that would seem to be to either crash or do nothing if this method is called when this is disposed.

Now, if there was some way the timer could be disposed and still tick, and some other way it could get disposed, it might make sense to check whether IT was disposed...