Getting Build from QueueReference

Hi All¡
First of all sorru for my ignorance, I´m new dealing with jenkins-api. I´m going to explain my doubt:

I have a process that call jenkins in order to launch a build:

` JenkinsServer jenkins = new JenkinsServer(new URI(
"https://jenkins"), "xxxxx",
"xxxxx");
Map params = new HashedMap();
params.put("svnPath","xxxxxx@44117");

QueueReference queue = jenkins.getJob("JARGENERATION").build(params);`

Now I´ve to fetch the artifact and I´m trying to get it by:

`QueueItem item = jenkins.getQueueItem(queue);
BuildWithDetails build = jenkins.getBuild(item).details();`

I guest that the problem is that the build is in process, but by debugging always get NullPointer inside the API.

Some workround of how to fetch the artifact from a build recently launched?

Thanks in advance