Thanks, Valter Henrique, with your tip i managed to realise, that i simply entered incorrect path to this image. In one of my tries i use
String pathToImageSortBy = "resources/testDataIcons/filling.png";
ImageIcon SortByIcon = new ImageIcon(getClass().getClassLoader().getResource(pathToImageSortBy));
But correct way was use name of my project in path to resource
String pathToImageSortBy = "nameOfProject/resources/testDataIcons/filling.png";
ImageIcon SortByIcon = new ImageIcon(getClass().getClassLoader().getResource(pathToImageSortBy));