GWT RPC : remote call

Hi all, 

I got an error in my GWT application, when i call a server side methode to get Data in order to display them in a pieChart (GWT visualization), here is the code : 

Server Side : 

public class SpdhServiceImpl extends RemoteServiceServlet implements

SpdhService {

/**

* 

*/

private static final long serialVersionUID = 1L;

@Override

public  Data_entity[] getEmployeesByCity()  {

Data_entity[] d_entity = new Data_entity[5] ;

d_entity[0].setCity("Casablanca");

d_entity[0].setCount(23);

return  d_entity ;

}

}

Errors:

First part : 

PM com.google.appengine.tools.development.ApiProxyLocalImpl log

GRAVE: javax.servlet.ServletContext log: Exception while dispatching incoming RPC call

com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract com.spt.sprd.shared.Data_entity[] 

com.spt.sprd.client.SpdhService.getEmployeesByCity() throws java.lang.IllegalArgumentException' threw an unexpected exception: java.lang.NullPointerException

at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure(RPC.java:389)

Second part : 

Caused by: java.lang.NullPointerException

at com.spt.sprd.server.SpdhServiceImpl.getEmployeesByCity(SpdhServiceImpl.java:25)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)

Thanks a lot for your help.

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