Bug 1112135 - [Messages] When starting as an activity/notification/, delay threads rendering until the right panel is open. r=julien

You can simply store the resolved promise you return in https://github.com/mozilla-b2g/gaia/pull/26919/files#diff-0da4811b0d0ac56a9fa128ffbb83b9d2R914 and use it here. At least you're sure that the "then" callbacks are called in the order the `then` have been called.