Is My Null Pointer Exception Due To Threading?

When my code hits the if statement it immediately throws the error

Exception in thread "Thread-4" java.lang.NullPointerException
    at sample.Controller.getSiteLocation(Controller.java:125)
    at sample.Controller.access$000(Controller.java:30)
    at sample.Controller$1.run(Controller.java:72)
    at java.lang.Thread.run(Thread.java:748)

The only thing i can think of is due to it being in a diff thread, because the checkbox is checked by default.

@FXML
CheckBox cboxOnSiteSearch;

@FXML
public void initialize() {
    cboxOnSiteSearch.setSelected(true);
}
@FXML
private void QueryLocations(ActionEvent event)
{      

    Thread th = new Thread(new Runnable() {

        @Override
        public void run() {
            Controller obj = new Controller();
              Set<String> result = obj.getSiteLocation();
            try {
                Thread.sleep(0);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    });
    th.setDaemon(true);
    th.start();
} 

private Set<String> getSiteLocation() {
    if (cboxOnSiteSearch.isSelected())
    {
        //more code
    }
    //more code
}

What do I need to change so that this code does not produce an error?