Getting GWT RPC Serialization Error while populating liststore of ComboBox - gwt

Question

I am using GWT 2.9 and I am getting below error
SEVERE: Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'com.xyz.nos.model.NosOptionModel' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = com.xyz.nos.model.NosOptionModel#6cad8f33
SEVERE: Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'com.xyz.nos.model.NosOptionModel' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = com.xyz.nos.model.NosOptionModel#5fc5e111
SEVERE: Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'com.xyz.nos.model.NosCaseDetailModel' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = com.xyz.nos.model.NosCaseDetailModel#af3642f
SEVERE: Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'com.xyz.nos.model.CountryGroupModel' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = com.xyz.nos.model.CountryGroupModel#51987007
SEVERE: Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'com.xyz.nos.model.NosUser' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = com.xyz.nos.model.NosUser#28bbfa7f
I am doing GWT RPC for populating liststore of combobox and getting above exception while performing it from EntryPoint class. All the above classes are Pojo's and all the fields of these classes are Serializable
I have tried all possible solutions given in GWT - RPC SerializationException
I have defined my service as below
public interface INamedComboListDataProvider<M> extends RemoteService {
public List<M> load(String name);
}
I also have checked my serializationWhitelist and it only contain following 25 classes
{class com.google.gwt.user.client.ui.TreeListenerCollection=true, class java.lang.String=true, class com.google.gwt.user.client.rpc.IncompatibleRemoteServiceException=true, class com.google.gwt.user.client.ui.FocusListenerCollection=true, class com.google.gwt.user.client.rpc.RpcTokenException=true, class java.util.LinkedList=true, class java.lang.RuntimeException=false, class com.google.gwt.user.client.ui.LoadListenerCollection=true, class com.google.gwt.user.client.ui.ClickListenerCollection=true, class com.google.gwt.user.client.ui.FormHandlerCollection=true, class java.lang.Throwable=false, class com.google.gwt.user.client.ui.KeyboardListenerCollection=true, class com.google.gwt.user.client.ui.MouseListenerCollection=true, class com.google.gwt.user.client.ui.TabListenerCollection=true, class java.util.Vector=true, class com.google.gwt.user.client.ui.ChangeListenerCollection=true, class com.google.gwt.user.client.ui.ScrollListenerCollection=true, class com.google.gwt.user.client.ui.PopupListenerCollection=true, class com.google.gwt.user.client.ui.MouseWheelListenerCollection=true, class java.util.Arrays$ArrayList=true, class java.lang.Exception=false, class java.util.ArrayList=true, class java.util.Collections$EmptyList=true, class java.util.Collections$SingletonList=true, class com.google.gwt.user.client.ui.TableListenerCollection=true}
I also have added project folder structure below
client
model
service
services
util
Common.gwt.xml
Main.gwt.xml
Can anybody guide me where I am going wrong and why does the serializationwhilteList does not contain the pojo models ?