Fix for crashes when nav items tapped while views are transitioning.

There had been code to debounce the "W" menu, but another crash was
discovered: Tap edit pencil, then tap "X" icon and "W" menu icon in
quick succession. CRASH! This patch is a generic solution which
handles both of these crashes and should prevent any similar crashes
elsewhere.

Change-Id: I838f101878fe5d1337d21a64f974a3a03c595609