JS: add null guard in window resize
JS: add null guard in window resize

The control panel resize can get called on startup before we've got a
JaamSimModel created, avoid an exception backtrace in that case.

Signed-off-by: Harvey Harrison <eadbd6b462bf3c97df0300a934c12bc2e5d1fe51@gmail.com>

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