JSF list depending on selected value returning nullPointer

In my jsf page, in a dialog, I have a list of EntityB which depends on selected EntityA in a datatable.
When I first load the page it's giving me nullPpointer exception because nothing is selected in the first place. Can anyone tell me how to prevent this?

EDIT: I added an actionlistener to the link who open the dialog
and getting this error:

>Cannot convert DemandesDocsAdministratif type DemandesDocsAdministratif to class javax.faces.event.ActionEvent

JSF:

<p:commandLink value="#{demande.idDemandeDocAdministratif}"
					oncomplete="PF('dlg2').show()" process="@this"
					update=":form:pg" actionListener="#{gestionDemandesMB.fillEntityB}">
					<f:setPropertyActionListener
						target="#{gestionDemandesMB.SelectedEntityA}" value="#{demande}" 
						/>
				</p:commandLink>
<form>
<datatable>
</datatable>
    <p:dialog>
    	<p:selectOneMenu id="Signataires"
    							value="#{gestionDemandesMB.entityB}">
    							<f:selectItems value="#{gestionDemandesMB.listEntityB}"
    								var="sign" itemLabel="#{sign.libRole}"
    								itemValue="#{sign.idPoste}" />
    						</p:selectOneMenu>
    </p:dialog>
</form>

Bean:

public List<EntityA> getListEntityB() {
	if ( selectedentityA != null ){
	return entityBService.ListByentityA(selectedEntityA)
			; } else {
	return Collections.emptyList() ; }

Bean listener that I'm working with now:

public void fillSignataires(ActionEvent event)
	{
listB = entityBService.ListByentityA(selectedEntityA)

		signaRender = true ;
	}

this is the getter of entity B list, I'm looking for a way to either get an empty list or call only when I open dialog.

You could use rendered="#{not empty bean.list}" to prevent the rendering of the selectOnMenu until you have the object filled.

You can also add a f:selectItem with a "Select" label and empty itemValue such as itemValue = " ". By the way, never have a null list, list shouldn't be null but it can be empty. It's the best practice. So you can initialize your listEntityB as an empty list at post construct of your bean.

    <p:selectOneMenu id="Signataires"
                            value="#{gestionDemandesMB.entityB}">
                            <f:selectItem  itemLabel="Select" itemValue="" />
                                var="sign" itemLabel="#{sign.libRole}"
                            <f:selectItems value="#{gestionDemandesMB.listEntityB}"
                                var="sign" itemLabel="#{sign.libRole}"
                                itemValue="#{sign.idPoste}" />
                        </p:selectOneMenu>