Java API getStates() Problem

Hello,
I'm currently working on my project for univercity and I decided to use OpenSky API, which I'm very thankful for

But I have a problem in refreshing flight state.

My code works like in the example in documentation:
1. I create an OpenSkyApi object in main class:
OpenSkyApi api = new OpenSkyApi("NICKNAME", "PASSWORD");

...and I run getStates() method with given bounding box, so it works fine and i retrieve some current flights.

Then, I create a separate Thread for every flight, and in later time, every Thread has to refresh its states of flight (this is my project for concurrent programming, so I have to show the teacher that i can use Threads, but this is not the problem here)

2. I send "api" object to my second class called "Flight" (which is a new Thread) to the controller method and I'm trying to retrieve states there:
OpenSkyStates states = api.getStates(0, myicao24 );

where "myicao24" is String object with an icao24 inside, like: "06a086"

but when I compile that code, compiler says that there should be a String Array, so i make it like:
OpenSkyStates states = api.getStates(0, new String[] {myicao24} );
...as it would be an Array with one String inside, so compiler says that it's OK, but the getStates() method returns nullPointerException, even if I check this flight by executing getStates() method with "null" parameter for icao24 and given bounding box and this icao24 is visible as a result :/

What am I doing wrong? How to get states for only one exact icao24 in Java?
Please help me