javafx NullPointerException in the for loop

Question

I'm placing images inside hbox where the hbox located inside the vbox

FileInputStream seats_fileInputStream = new FileInputStream("seat.png");
    Image seats_image = new Image(seats_fileInputStream,50,50,false,false);

    ImageView[] seats = new ImageView[30];

    for(int i = 0;i<30;i++){
    seats[i] = new ImageView(seats_image);
    }

    HBox seatsRaw_hbox[] = new HBox[4];
    VBox seatsLine_vbox = new VBox();

    for(int i=0;i<5;i++){
             for(int j=0;j<6;j++){
           seatsRaw_hbox[i].getChildren().addAll(seats[j]);             
             }

           seatsLine_vbox.getChildren().addAll(seatsRaw_hbox[i]);

    } 

in should display 6 images in each hbox where there is 5 hboxes it total and place them into the vbox. But I keep getting
java.lang.NullPointerException runtime error in the line

   seatsRaw_hbox[i].getChildren().addAll(seats[j]);