TypeError: Cannot read properties of undefined (reading 'then')

[sentry]

In forceServerSync, if the client already has an active sync, the return value of debouncedSyncChanges.flush() will be undefined.

Sentry Issue: STUDIO-FX3

TypeError: Cannot read properties of undefined (reading 'then')
  at session.actions.logout (./shared/vuex/session/index.js:121:16)
...
(8 additional frame(s) were not displayed)

[rtibbles]

Fixed in #3934