adding a bookmarks folder makes the browser unresponsive

[diracdeltas]

Test plan

#9746 (comment)

---

master and 0.18.x are failing with

  1) Syncing bookmarks "before all" hook:
     WaitUntilTimeoutError: element ([data-l10n-id="done"]) still not visible after 10000ms
      at isVisible("[data-l10n-id="done"]") - waitForVisible.js:37:22
  2) Syncing bookmarks from an existing profile "before all" hook:
     WaitUntilTimeoutError: element ([data-l10n-id="done"]) still not visible after 10000ms
      at isVisible("[data-l10n-id="done"]") - waitForVisible.js:37:22

it turns out this is because adding a bookmarks folder causes a fatal browser error (see comment below).

STR:

1. open fresh browser profile
2. right-click to add a bookmark folder. click 'done'.
3. the browser is unresponsive and you can't open any more tabs or change tabs

[diracdeltas]

Uncaught TypeError: Cannot read property 'get' of undefined
    at mergeProps (localhost:8080/gen/app.entry.js:75084)
    at buildPropsImpl (localhost:8080/gen/app.entry.js:73319)
    at ReduxComponent.buildProps (localhost:8080/gen/app.entry.js:73391)
    at ReduxComponent.checkForUpdates (localhost:8080/gen/app.entry.js:73356)
    at WindowStore.EventEmitter.emit (localhost:8080/gen/app.entry.js:37228)
    at WindowStore.emitChanges (localhost:8080/gen/app.entry.js:63842)
    at doAction (localhost:8080/gen/app.entry.js:64475)
    at callbacks.forEach (localhost:8080/gen/app.entry.js:41083)
    at Array.forEach (<anonymous>)
    at AppDispatcher.dispatchToOwnRegisteredCallbacks (localhost:8080/gen/app.entry.js:41082)