[FIX] Fix selected message count on mail messages sidenav #12376
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Closes #12363
BEFORE FIX:
![undefined-messages mov](https://user-images.githubusercontent.com/6295044/47234294-5bea1e80-d3a3-11e8-8811-a92c17634e24.gif)
AFTER FIX:
![fixed-messages-email mov](https://user-images.githubusercontent.com/6295044/47234299-5f7da580-d3a3-11e8-9d10-b08e7f2dc5a5.gif)