Xap packaging failed. Object reference not set to an instance of an object

I'm getting this error message on my PC when I try to build  Silverlight App:

Xap packaging failed. Object reference not set to an instance of an object 

I'm using Dropbox. On my laptop, It works perfectly fine and I can debug etc...

Please tried many things such as:

• Delete obj/Debug Folder
• Check for files which are missing on the Solution Explorer