nightly build #377

To reproduce:

1. Add a Container and a Button
2. Replace text of the Button with i18n key, e.g. %button
3. Set i18n resource properties file. And there is not a key "button" in the file.
4. Now a warning icon appear in front of the Button on Hierarchy Panel. Place mouse on the icon, there is a blank tooltip displayed.

The similar tooltip on the information bar of Content Panel works fine.

snapshot is attached.