Small fixes

[Cimbali]

I was going over the code to look at writing up some PRs and stumbled across 2 small easily corrected issues:

• a Promise’s resolve() being called twice, which is meaningless.
  Note that I’ve checked that using else for fallback/function exit is OK per style, see e.g. https://github.com/mozilla/multi-account-containers/blob/60b40a2d9f609572bd55fd59f7206ff4680365ef/src/js/background/assignManager.js#L16-L20
• A function call to getByContainer() which doesn’t exist (there is a getAssignedSites() which in turn calls _getByContainer())