android gen错误：/albume/gen already exists but is not a source folder.

错误：/albume/gen already exists but is not a source folder. 

Try this, it helped me:

1. Right click on the project and go to "Properties"
2. Select "Java Build Path" on the left
3. Open "Source" tab
4. Click "Add Folder..."
5. Check "gen" folder and click Ok and Ok again
6. Again right click on the project and in the "Andriod Tools" click on "Fix Project Properties"