java.lang.NullPointerException

[oll3i]

Exception in thread "AWT-EventQueue-0" java.lang.NullP ointerException
at Zad21.Cennik.ge tCenna(Cennik.j ava)
at Zad21.Kwiaciarn iaView$1.action Performed(Kwiac iarniaView.java )
at javax.swing.Abs tractButton.fir eActionPerforme d(Unknown Source)
at javax.swing.Abs tractButton$Han dler.actionPerf ormed(Unknown Source)

package Zad21;

import java.util.HashMap;
import java.util.Map;

public class Cennik {
	 private static Cennik instance = null;
	 Map<String,Double> cennik = new HashMap<String,Double> ();

	   private Cennik() {  // prywatny konstruktor
	   }

	   public static Cennik getInstance() {
	      if(instance == null) {
	         instance = new Cennik();
	         
	      }
	      return instance;
	   }
	   
public void setCennik(String nazwaKwiat,double cena){	   
	cennik.put(nazwaKwiat,cena);   
}

public  Map<String,Double> getCennik(){ return cennik;}

public double getCenna(String nazwaKwiata){ return cennik.get(nazwaKwiata);}
}

in some other class

m_add.addActionListener( new ActionListener() {
    public void actionPerformed(ActionEvent e) {
    	 Cennik c= Cennik.getInstance();
    	
    	 
    	 int a=Integer.parseInt(m_amount.getText());
    	 table.addRow((String)kwiatyList.getSelectedItem(),a,c.getCenna((String)kwiatyList.getSelectedItem()));
    }
  });

i want to get the value of selected item in combobox when a button is clicked
String)kwiatyLi st.getSelectedI tem(), and when a user enters some input into JTextField then take the both values when a button is clicked and add the entries to a table
i get the nullpointerexce ption in c.getCenna((Str ing)kwiatyList. getSelectedItem ()));

thank u

[oll3i]

i solved it thank u