Primefaces GMap OverlaySelect event and poping up an output panel while clicking

I've a probleme in primeface 4. I have a map and I want to see an output panel while clicking on a marked position in the map

<p:gmap  model="#{addMarkers.emptyModel}" id="gmap" streetView="false"  navigationControl="true" mapTypeControl="true"  draggable="true" center="26.959512, -12.821044" zoom="8" type="ROADMAP" style="width:100%;height:500px">
           <p:ajax  event="overlaySelect" listener="#{addMarkers.onMarkerSelect}" id="open1"  update="messages"/>
           <p:gmapInfoWindow  id="infoWindow">
            <p:outputPanel style="text-align: center;  margin: auto">
                <p:commandButton ajax="true" type="button" value="open map"  onclick="PF('dlg').show();" />

When i do refresh the page, the problem seems to be solved and surprisingly I get my output panel ! After debuging I found that I have a little problem in my ManagedBeans

public void onMarkerSelect(OverlaySelectEvent event)  {
    marker = (Marker) event.getOverlay();
     FacesContext.getCurrentInstance().addMessage(null, new FacesMessage(FacesMessage.SEVERITY_INFO, "scanning the materials", "Site :"+ marker.getTitle() ));

}

Before refreshing I'm getting: event overlay type is unknown and the value is NULL After refereshing I'm getting event overlay type is "Marker" and the value is #440