使用WindowsBuilder进行可视化设计时不能预览界面

今天用windowsBuilder设计界面，本来想自己写代码，然后再在可视化窗口中预览，看看效果，可是提示"WindowBuilder was not able to show the
GUI”的错误。google了一下，发现原来是布局不对造成的：

I tried creating a very simple project in which I created a layout manager named SomeLayout.
I copied into it the exact code from FillLayout, and I also created a SomeLayoutInfo class with helper classes, which I modeled exactly after the FillLayoutInfo and associated from the WB source tree. It doesn't seem to be working the way I expected. I have
attached a very simple sample Eclipse project that highlights the problem that I am seeing (note that the classpath will need to be massaged a bit to point to your local SWT version and WB installation directory).

In the sample project I have psoted, when I click on the Design tab for MainWindow.java, I get
a "WindowBuilder was not able to show the GUI" message. If I comment out line 47:

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// shell.setLayout(new SomeLayout(SWT.HORIZONTAL));