Android: Unknown Error NullPointerException “Android Packaging Problem”

I got the above error when I was renaming some Library projects, there was no obvious code problem, so it was difficult to figure out what is wrong.

I tried the usual following:

1. Checked all library projects in Properties > Android (they were good)
2. Project > Clean..
3. Restart Eclipse

None of the above worked, finally I had to go to .classpath file and deleted the classpath tags and re-added the Reference Libraries, after the project rebuilt the problems cleared.

<classpathentry kind="src" path="Picasa_Library_src"/>