用swt后在design窗口显示Unknown Window Type

[HLandHB]

package org.hhkj.hhb;

import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class HelloWorld
{
public static void main(String[] args)
{
final Display display = Display.getDefault();
final Shell shell = new Shell();
shell.setSize(500, 375);
shell.setText("SWT Application");

shell.layout();
shell.open();
while (!shell.isDisposed())
{
if (!display.readAndDispatch())
display.sleep();
}
}
}

想通过design添加一个文本框组件,单击design却出现Unknown Window Type.design的其它地方全是空的(没有组件列表,没有属性列表等)