display message after page load, if a combobox is empty - jsf

Question

I tried populating the collection in the #PostConstruct method, then a NPE is thrown.
Also tried redirecting, but I get IllegalStateException: Response already committed.
this.networks = this.getNetworks();
if (this.networks.isEmpty()) {
JsfUtils jsfUtils = new JsfUtils();
jsfUtils.displayMessage("WARN", "No network inserted!", "Insert a network!");
}
Then I tried using FacesMessage directly inside the getter method. The message isn't shown, though the method is being executed since I see the message in the console.
public List<Network> getNetworks() {
if (this.networks == null) {
networks = networkBean.findWithNamedQuery("Network.findAll");
if (networks.isEmpty()) {
FacesContext fc = FacesContext.getCurrentInstance();
fc.addMessage(null, new FacesMessage(FacesMessage.SEVERITY_WARN,"Insert a network!",""));
System.out.println(">>>>>>>>>>>> Insert a Network!");
}
}
return networks;
}
Using the same structure as the first in a #PostLoad method also doesn't display any message.
Page code:
<ui:composition xmlns="http://www.w3.org/1999/xhtml" xmlns:c="http://xmlns.jcp.org/jsp/jstl/core" xmlns:f="http://xmlns.jcp.org/jsf/core" xmlns:h="http://xmlns.jcp.org/jsf/html" xmlns:jsf="http://xmlns.jcp.org/jsf" xmlns:p="http://primefaces.org/ui" xmlns:pt="http://xmlns.jcp.org/jsf/passthrough" xmlns:ui="http://xmlns.jcp.org/jsf/facelets"
template="/WEB-INF/tpl/template1.xhtml">
<ui:define name="title">Add Stablishment</ui:define>
<ui:define name="header">Add Stablishment</ui:define>
<ui:define name="content">
<h:form id="form" enctype="multipart/form-data">
<p:growl id="growl" autoUpdate="true" showDetail="true" sticky="true" globalOnly="true" />
<h:panelGrid columns="3" cellpadding="5">
<p:outputLabel for="categories" value="Category:" />
<p:selectOneMenu id="categories" value="#{stablishmentMB.category.id}">
<f:selectItem itemValue="" itemLabel="-- Select --" noSelectionOption="true" />
<f:selectItems value="#{stablishmentCategoryMB.categories}" var="category" itemLabel="#{category.name}" itemValue="#{category.id}" />
</p:selectOneMenu>
<p:message for="categories" />
<p:outputLabel for="networks" value="Network:" />
<p:selectOneMenu id="networks" value="#{stablishmentMB.network.id}">
<f:selectItem itemValue="" itemLabel="-- Selecione --" noSelectionOption="true" />
<f:selectItems value="#{networkMB.networks}" var="network" itemLabel="#{network.tradeName}" itemValue="#{network.id}" />
</p:selectOneMenu>
<p:message for="networks" />
<p:outputLabel for="cnpj" value="CNPJ:" />
<p:inputMask id="cnpj" pt:placeholder="99.999.999/9999-99" mask="99.999.999/9999-99" value="#{stablishmentMB.stablishment.cnpj}" required="true" />
<p:message for="cnpj" />
<p:outputLabel for="companyName" value="Company Name:" />
<p:inputText id="companyName" value="#{stablishmentMB.stablishment.companyName}" required="true" />
<p:message for="companyName" />
<p:outputLabel for="tradeName" value="Trade Name:" />
<p:inputText id="tradeName" value="#{stablishmentMB.stablishment.tradeName}" required="true" />
<p:message for="tradeName" />
<p:outputLabel for="zipCode" value="Zip Code:" />
<p:inputMask id="zipCode" mask="99999-999" value="#{stablishmentMB.zipCode.number}" required="true">
<p:ajax event="blur" listener="#{stablishmentMB.findByZipCode}" update="street neighbourhood city state" />
</p:inputMask>
<p:message for="zipCode" />
<p:outputLabel for="state" value="State:" />
<p:inputText id="state" value="#{stablishmentMB.state.abbreviation}" required="true" />
<p:message for="state" />
<p:outputLabel for="city" value="City:" />
<p:inputText id="city" value="#{stablishmentMB.city.nome}" required="true" />
<p:message for="city" />
<p:outputLabel for="neighbourhood" value="Neighbourhood:" />
<p:inputText id="neighbourhood" value="#{stablishmentMB.neighbourhood.nome}" />
<p:message for="neighbourhood" />
<p:outputLabel for="street" value="Street:" />
<p:inputText id="street" value="#{stablishmentMB.street.name}" required="true" />
<p:message for="street" />
<p:outputLabel for="number" value="Nº:" />
<p:inputText id="number" value="#{stablishmentMB.stablishment.number}" required="true" />
<p:message for="number" />
<p:outputLabel for="complement" value="Complement:" />
<p:inputText id="complement" value="#{stablishmentMB.stablishment.complement}" />
<p:message for="complement" />
<p:outputLabel for="telephone" value="Telephone:" />
<p:inputMask mask="(99) 9999-9999" id="telephone" value="#{stablishmentMB.stablishment.telephone}" />
<p:message for="telephone" />
<p:outputLabel for="contact" value="Contact:" />
<p:inputText id="contact" value="#{stablishmentMB.stablishment.contact}" />
<p:message for="contact" />
<p:outputLabel for="email" value="E-mail:" />
<p:inputText id="email" value="#{stablishmentMB.stablishment.email}" />
<p:message for="email" />
<p:outputLabel for="site" value="Site:" />
<p:inputText id="site" value="#{stablishmentMB.stablishment.site}" />
<p:message for="site" />
<p:outputLabel for="latitude" value="Latitude:" />
<p:inputText id="latitude" value="#{stablishmentMB.stablishment.latitude}" />
<p:message for="latitude" />
<p:outputLabel for="longitude" value="Longitude:" />
<p:inputText id="longitude" value="#{stablishmentMB.stablishment.longitude}" />
<p:message for="longitude" />
<p:commandButton action="#{stablishmentMB.save}" value="Save" update="#form" />
</h:panelGrid>
</h:form>
</ui:define>

Answer 1

You should remove your code from the getter because a getter is not mean to do such staff and always #PostConstruct will be executed before any getter on your bean!
A quick fix to your problem is to move <p:growl> to the end of page this will make your bean construction happens before the renderer try to render <p:growl>.