Primefaces update the panel before f:setPropertyActionListener

Question

I have the following managed bean which has a selectedElement property and some components in a primefaces view binded to it inside an auto update outputpanel, i have a commandButton and f:setPropertyActionListener to set the selectedElement then update the panel, i get Target Unreachable, 'null' returned null: javax.el.PropertyNotFoundException: i think the update proess fires before setting the property , i have another view in different page do the same procedure without errors ! the managed bean code is:

package boddooo.service;

import java.util.List;

import javax.faces.bean.ManagedBean;
import javax.faces.bean.SessionScoped;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;
import javax.transaction.UserTransaction;

import boddooo.entity.Element;

@ManagedBean(name="ob")
@SessionScoped
public class objectsbuilder {

UserTransaction ut=null;

@PersistenceContext(unitName="game")
private EntityManager em;

Context icontext=null;
public objectsbuilder() throws NamingException{
    icontext=new InitialContext();
    ut=(UserTransaction)icontext.lookup("java:comp/UserTransaction");
}

private List<Element> elements;

private Element selectedElement;

public Element getSelectedElement() {
    return selectedElement;
}

public void setSelectedElement(Element selectedElement) {
    this.selectedElement = selectedElement;
}

@SuppressWarnings("unchecked")
public void loadElements(){
    Query q=em.createNamedQuery("Element.findAll");
    elements=(List<Element>)q.getResultList();
}

public List<Element> getElements(){
    return elements;
}

}

and the view is :

<p:layout fullPage="true">
    <p:layoutUnit position="west" header="Elements">
        <p:dataGrid value="#{ob.elements}" var="element" style="padding:0px;text-align:center" columns="5">
            <p:column>
                <p:commandButton value="" style="margin:0px;padding:0px;width:40px;height:40px;background-image: url('resources/images/objects/#{element.name}.png');background-size: 40px 40px;">
                    <f:setPropertyActionListener value="#{element}" target="#{ob.selectedElement}"></f:setPropertyActionListener>
                </p:commandButton>
            </p:column>
        </p:dataGrid>
    </p:layoutUnit>
    <p:layoutUnit position="center">
        <p:outputPanel autoUpdate="true">
            <p:panelGrid columns="2">
                <p:outputLabel value="Name"></p:outputLabel>
                <p:inputText value="#{ob.selectedElement.name}"></p:inputText>
                <p:outputLabel value="Type"></p:outputLabel>
                <p:inputText value="#{ob.selectedElement.type}"></p:inputText>
                <p:outputLabel value="Class"></p:outputLabel>
                <p:inputText value="#{ob.selectedElement.class_}"></p:inputText>    
                <p:outputLabel value="Upgradable"></p:outputLabel>
                <p:inputText value="#{ob.selectedElement.upgradable}"></p:inputText>
                <p:outputLabel value="Duplicated"></p:outputLabel>
                <p:inputText value="#{ob.selectedElement.duplicated}"></p:inputText>
                <p:outputLabel value="Period"></p:outputLabel>
                <p:inputText value="#{ob.selectedElement.period}"></p:inputText>                                    
            </p:panelGrid>              
        </p:outputPanel>        
    </p:layoutUnit>
</p:layout>