java - JOptionPane not showing up (null parameter, called from inner class) -

i new java, trying make simple application , there no way can joptionpane show correctly , guess missing something:

here's code:

import java.awt.color; import java.awt.window; import java.awt.event.actionevent; import java.awt.event.actionlistener;  import javax.swing.jframe; import javax.swing.jmenu; import javax.swing.jmenubar; import javax.swing.jmenuitem; import javax.swing.joptionpane; import javax.swing.jpanel;   public class frame  extends jframe {            private static final long serialversionuid = 1l;     final static int width = 400;     final static int height = 400;      public frame()     {         super("convert dxf alpha ver. - survey apps 2014");         setdefaultcloseoperation(exit_on_close);         setsize(width, height);         setresizable(false);         setlocation(100, 100);         setbackground(color.white);         setvisible(true);         welcomepanel welcomepanel = new welcomepanel(this);         add(welcomepanel);               }      public void createmenupanel()     {         menupanel menu = new menupanel();         setjmenubar(menu.createmenu(this));     } }  class menupanel extends jpanel implements actionlistener {     private jmenuitem open,save,close;     private jmenu file,about;     private frame frame;     private static final long serialversionuid = 1l;      public jmenubar createmenu(frame frame)     {         this.frame = frame;         jmenubar menubar = new jmenubar();                   file = new jmenu("file");         = new jmenu("about");                  menubar.add(file);         menubar.add(about);                  open = new jmenuitem("open");         save = new jmenuitem("save");         close = new jmenuitem("close");          file.add(open);         file.add(save);         file.addseparator();         file.add(close);          open.addactionlistener(this);         save.addactionlistener(this);         close.addactionlistener(this);         about.addactionlistener(this);          return menubar;     }      public menupanel()     {         setvisible(true);         setopaque(true);         setbackground(color.white);         setsize(window.width,window.height);     }         @override     public void actionperformed(actionevent e)     {     object source = e.getsource();          if(source == open)         {             frame.dispose();         }          if(source == save)         {                    }          if(source == close)         {                }         if(source == about)         {             joptionpane.showmessagedialog(frame, "easydxfcreator - alpha version");         }     }  } 

(i skipped welcomeframe because it's welcome screen disappears after mouse click)

jmenu not operate in way. jmenu events need implement menulistener instead of actionlistener. actionlistener jmenuitem.

hope helps.