Timeline: Check various members of TimelineOverviewRenderer for sanity

In some transitional states we could end up without a zoomControl or a
model. Make sure we won't access it then.

Change-Id: I20b2363aae5fd860e97b393a65be20da26cec437
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>