Bug 1633019 - Crash in [@ mozilla::PresShell::EventHandler::MaybeFlushThrottledStyles], r=masayuki a=RyanVM

GetViewManager() may return null if the relevant presshell is being destroyed.
This is a guess fix, but the crash reports seem to hint about this issue.

Differential Revision: https://phabricator.services.mozilla.com/D161587