fix exception when opening empty folder and file open
Update CONTRIBUTING.md

Add section about "Starter Bugs".
Merge branch 'master' of https://github.com/adobe/brackets into Issue-2509
Merge remote-tracking branch 'upstream/master' into WebsiteDeveloper/ProjectManager
Merge remote-tracking branch 'upstream/master' into WebsiteDeveloper/ProjectManager
Updated ProjectManager.js
Changed to use no-focus class
Merge pull request #2879 from WebsiteDeveloper/Issue-2878

Fixed: Issue #2878 Expanding/collapsing a folder tree node steals focus from editor