JS: add null checks to ObjectSelector
JS: add null checks to ObjectSelector

When a new model is opened it is possible for the Object Selector to
receive an update call before the Simulation object has been set up
correctly.

Signed-off-by: Harry King <409587b9e6671aa0763646191d292852dc49a658@gmail.com>

jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim