fix console warnings on RCE event listener
fix console warnings on RCE event listener

when loading the RCE you may see console warnings about failures
adding an event listener when an expected element does not exist

Change-Id: Ib9ecc1500a254f913d610d0effd7af278d2353d0
Reviewed-on: https://gerrit.instructure.com/209533
Tested-by: Jenkins
Reviewed-by: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms