Dropbox is reaching out to users to let them know of an issue with OS X Yosemite's Finder integration. Dropbox uses Yosemite's official Finder integration, which might cause a crash when users open or "Save As" a file, or when you first save a file.
Luckily, Apple has already issued a fix for this issue. Users should download OS X 10.10.1, which was released earlier this week. In addition to fixing this issue, 10.10.1 also addresses Wi-Fi issues, remote computer connection issues, and more. You can download OS X 10.10.1 by going to the Updates section in the Mac App Store.