The reason is that comboUri variable in the statement 
protected URI getComboUri() { return comboUri; } (line number 81)
refers to com.ibm.jaggr.core.impl.transport.AbstractDojoHttpTransport.comboUri which is by default null and this test case fails.
I think this is as a result of the refactoring to introduce AbstractDojoHttpTransport in jaggr.core package.