Xap packaging failed. Object reference not set to an instance of an object
The problem lies in your startup object in your Application. I have met this error to many times.
It usually happens when I change the name of my class/namespace Application class. Or it might even be conflicting with your dropbox locations.
To Fix this.
- Right click your main project or Alt+Enter
- Go to the silverlight tab and set your startup object
You should check, in every folder of your project, for missing files or wrong references.
You can get this problem just with a single missing image, if its Build Action property is set to Content or similar...
Check your files and then Rebuild your project!
I am adding this reply because although the problem with app icons is covered in brief in some of the answers above, I wanted to give exact steps since it just happened to me. I got this error and then found this web page:
Xap Packaging Error
It turned out that my App icon file name was wrong due to a rename. I double-clicked on WMAppManifest.xml, found in the Properties folder, to open the editor for that file. I simply used the button to update the App Icon, re-found the file and selected it, and the error went away after rebuilding the project.
This can happen if a file is removed from the disk itself, but not from the Solution through Visual Studio. Delete the file through Visual Studio (it should be marked with a little yellow hazard icon) and Rebuild.
Credit to Andreas Hammar via http://blog.jayway.com/2011/06/03/wp7-build-error-xap-packaging-failed/ - his solution worked for me.