Ivy Issue: Unresolved Dependencies - java.lang.NullPointerException at java.util.Hashtable.put()

You might encounter the following error while building your project and Ivy is unable to resolve dependencies.

[ivy:retrieve] 
[ivy:retrieve] :: problems summary ::
[ivy:retrieve] :::: WARNINGS
[ivy:retrieve]   ::::::::::::::::::::::::::::::::::::::::::::::
[ivy:retrieve]   ::          UNRESOLVED DEPENDENCIES         ::
[ivy:retrieve]   ::::::::::::::::::::::::::::::::::::::::::::::
[ivy:retrieve]   :: org.mortbay.jetty#jetty;6.1.26: java.lang.NullPointerException at java.util.Hashtable.put(Hashtable.java:514)
[ivy:retrieve]   ::::::::::::::::::::::::::::::::::::::::::::::
[ivy:retrieve] 
[ivy:retrieve] :: USE VERBOSE OR DEBUG MESSAGE LEVEL FOR MORE DETAILS

BUILD FAILED

It's probably because of some files, for example the checksum file, was corrupted so to solve the error, just delete the library from Ivy's cache so Ivy will download the library again when you rebuild. I used Ubuntu and I deleted it with the following command:

rm -rf ~/.ivy2/cache/org.mortbay.jetty/jetty