trying to implement implement chart with GWT

I am trying to implement this GWT-EXT chart. I have simply created a new GWT project and onModule I have pasted the source code as in this link and put my getext.jar in builpath.

When I run the project it never starts and it shows

 Plugin failed to connect to Development Mode server at 127.0.0.1:9997 

and in my development mode

 01:23:46.017 [ERROR] [gwtext] Line 41: No source code is available for type com.gwtext.client.widgets.Panel; did you forget to inherit a required module? 

Any idea why this is happening? Is there something I'm missing? This is my first time using GWT-EXT.

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