eclipse - WindowBuilder encountered unexpected internal error; Cannot cast? -

i'm trying run windowbuilder build swt apps. however, when click design tab, error:

the code i'm using produces above error basic default code when starting new project. same error occurs when try run actual swt project run when executed within java:

here's code:

package ch2; import org.eclipse.swt.*;  public class helloswt {      public static void main(string[] args) {         // todo auto-generated method stub         display display = new display();         shell shell = new shell(display);         shell.settext("hello world");         shell.setbounds(100, 100, 200, 50);          //layout         rowlayout rowlay = new rowlayout(swt.horizontal);         rowlay.justify = true;         rowlay.margintop = 20;         rowlay.marginbottom = 20;         //form layout         formdata formdata = new formdata();         formdata.right = new formattachment(100, -5);         formdata.bottom = new formattachment(100, -5);           //shell.setlayout(new filllayout(swt.vertical));         shell.setlayout(rowlay);          final label label = new label(shell, swt.center);         label.settext("hello inside world");             org.eclipse.swt.graphics.color red = new org.eclipse.swt.graphics.color(display, 255, 0, 0);         label.setforeground(red);          //button          final button button = new button(shell, 0);         button.settext("a button");             image img = new image(display, "c:\\users\\ra_stein\\pictures\\koala.png");         button.setimage(img);         button.setforeground(red);         button.pack();         button.addselectionlistener(new selectionadapter(){             public void widgetselected(selectionevent event){                 button.settext("a clicked button");                 label.settext("hello yourself");             }         });          text txt = new text(shell, swt.single);         txt.settext("text object");         txt.addverifylistener(new verifylistener(){                 public void verifytext(verifyevent event){                     event.doit = event.text.length() == 1 || character.isdigit(event.text.charat(0)); //set doit true if length of event.text (the text want put in) 1 or the first character digit.                 }             }         );          //table         final table table = new table(shell, swt.single | swt.border | swt.full_selection);         table.setheadervisible(true);         table.setlinesvisible(true);         tablecolumn column1 = new tablecolumn(table, swt.null);         column1.settext("name");         column1.pack();         tablecolumn column2 = new tablecolumn(table, swt.null);         column2.settext("age");         column2.pack();         tableitem item1 = new tableitem(table, swt.null);         item1.settext(new string[] {"dan", "43"});         tableitem item2 = new tableitem(table, swt.null);         item2.settext(new string[] {"john", "23"});         table.addselectionlistener(new selectionadapter() {             public void widgetselected(selectionevent event){                 tableitem[] selected = table.getselection();                 if (selected.length > 0){                     system.out.println("name: " + selected[0].gettext(0));                     system.out.println("age: " + selected[0].gettext(1));                 }             }          });          //tab folder         final tabfolder tabfolder = new tabfolder(shell, swt.border);         tabitem tabitem1 = new tabitem(tabfolder, swt.null);         tabitem1.settext("tab 1");         tabitem tabitem2 = new tabitem(tabfolder, swt.null);         tabitem2.settext("tab 2");          composite compo = new composite(tabfolder, swt.null);         tabitem2.setcontrol(compo);              //text hellotext = new text(shell, swt.center);         //hellotext.settext("hello swt");         //hellotext.pack();          shell.pack();         shell.open();          while(!shell.isdisposed()){             if (!display.readanddispatch())                 display.sleep();         }         display.dispose();       }  } 

and here's error: