NullPointerException problems

Question

I have a method that makes a JFrame visible, but i dont know why i cant run my program.

https://imgur.com/a/WcQ7SHw

 these are the error lines: 



   // here im calling to the method VentanaAlta to fill a combobox
//(errorline 21)

 public VentanaAlta() {
        initComponents();
        Main.llenarCombobox(cbPuesto);
    }



// here im trying to fill a combobox with Objetcs
//(errorline 41)-

public static void llenarCombobox(JComboBox comboPuesto){

      for(int x = 0; aPuestos.length > x; x++){

             comboPuesto.addItem(aPuestos[x].getNombrePuesto());
             comboPuesto.setSelectedItem(-1);
}


//this is a method that is called in the first line on the Main class (maybe //the problem is here, and yes, vAltas is a global variable)

//(errorline 95)

   public static void crearVentana(){
        vAltas = new VentanaAlta();
        vAltas.setVisible(true);             
    }


//(errorline 32)
llenarCombobox();

In the code you will see comments that shows the errorlines.

I have tried to be clear with the image and with the code comments, sorry for my englis i did my best.

Global Variables. aPersonas arrayList. cbPuesto is a combobox. llenarCombobox(cbPuesto); its the method to fill the combobox.