swing - Java GUI Blank and nullpointerexception -

i fixed when click on calculate button following:

this working except calculate button now, , important part. thank in advance help.

exception in thread "awt-eventqueue-0" java.lang.nullpointerexception     @ radio$calcbuttonlistener.actionperformed(radio.java:76)     @ javax.swing.abstractbutton.fireactionperformed(unknown source)     @ javax.swing.abstractbutton$handler.actionperformed(unknown source)     @ javax.swing.defaultbuttonmodel.fireactionperformed(unknown source)     @ javax.swing.defaultbuttonmodel.setpressed(unknown source)     @ javax.swing.plaf.basic.basicbuttonlistener.mousereleased(unknown source)     @ java.awt.component.processmouseevent(unknown source)     @ javax.swing.jcomponent.processmouseevent(unknown source)     @ java.awt.component.processevent(unknown source)     @ java.awt.container.processevent(unknown source)     @ java.awt.component.dispatcheventimpl(unknown source)     @ java.awt.container.dispatcheventimpl(unknown source)     @ java.awt.component.dispatchevent(unknown source)     @ java.awt.lightweightdispatcher.retargetmouseevent(unknown source)     @ java.awt.lightweightdispatcher.processmouseevent(unknown source)     @ java.awt.lightweightdispatcher.dispatchevent(unknown source)     @ java.awt.container.dispatcheventimpl(unknown source)     @ java.awt.window.dispatcheventimpl(unknown source)     @ java.awt.component.dispatchevent(unknown source)     @ java.awt.eventqueue.dispatcheventimpl(unknown source)     @ java.awt.eventqueue.access$200(unknown source)     @ java.awt.eventqueue$3.run(unknown source)     @ java.awt.eventqueue$3.run(unknown source)     @ java.security.accesscontroller.doprivileged(native method)     @ java.security.protectiondomain$1.dointersectionprivilege(unknown source)     @ java.security.protectiondomain$1.dointersectionprivilege(unknown source)     @ java.awt.eventqueue$4.run(unknown source)     @ java.awt.eventqueue$4.run(unknown source)     @ java.security.accesscontroller.doprivileged(native method)     @ java.security.protectiondomain$1.dointersectionprivilege(unknown source)     @ java.awt.eventqueue.dispatchevent(unknown source)     @ java.awt.eventdispatchthread.pumponeeventforfilters(unknown source)     @ java.awt.eventdispatchthread.pumpeventsforfilter(unknown source)     @ java.awt.eventdispatchthread.pumpeventsforhierarchy(unknown source)     @ java.awt.eventdispatchthread.pumpevents(unknown source)     @ java.awt.eventdispatchthread.pumpevents(unknown source)     @ java.awt.eventdispatchthread.run(unknown source)       import java.awt.*;     import java.awt.event.*;     import javax.swing.*;         public class radio extends jframe      {          private jpanel panel;         private jpanel buttonpanel;         private jtextfield base;         private jtextfield width;         private jradiobutton squarearea;         private jradiobutton parallelogramarea;         private final int window_width = 550;         private final int window_height = 550;         double ptotal;         double stotal;          public radio()         {             setsize(window_width, window_height);             settitle("area calculator");             setdefaultcloseoperation(jframe.exit_on_close);             setvisible(true);             buildpanel();             add(panel);          }           private void buildpanel()          {                 panel = new jpanel();             jlabel messagelabel1 = new jlabel("please enter base: ");             jtextfield base = new jtextfield(10);             jlabel messagelabel2 = new jlabel("please enter width: ");             jtextfield width = new jtextfield(10);             jradiobutton squarearea = new jradiobutton("choice 1", true);             jradiobutton parallelogramarea = new jradiobutton("choice 2");             buttongroup group = new buttongroup();             jbutton calcbutton = new jbutton("calculate");             calcbutton.setbackground(color.blue);             calcbutton.setforeground(color.pink);             calcbutton.addactionlistener(new calcbuttonlistener());             panel.add(messagelabel1);             panel.add(base);             panel.add(messagelabel2);             panel.add(width);             group.add(squarearea);             group.add(parallelogramarea);             panel.add(squarearea);             panel.add(parallelogramarea);             panel.add(calcbutton);         }             public static void main (string[] args)         {              radio radio = new radio();             radio.buildpanel();         }          private class calcbuttonlistener implements actionlistener         {              public void actionperformed(actionevent e)              {                   if (parallelogramarea.isselected());                 {                     ptotal = double.parsedouble(base.gettext()) * double.parsedouble(width.gettext());                     joptionpane.showmessagedialog(null, "the area is: " + ptotal);                 }                   if (squarearea.isselected())                 {                       stotal = double.parsedouble(base.gettext()) * double.parsedouble(width.gettext());                      joptionpane.showmessagedialog(null, "the area is: " + stotal);                 }             }         }     } 

i think buttonpanel null. can debug code line - by-line figure out of them null