working version, without exceptions

src/main/webapp/category.xhtml

@@ -12,11 +12,9 @@
 <h:body>
 
 
-    <h:form>
-        <h:selectOneMenu value="#{categoryBean.category}" onchange="this.form.submit()">
-            <f:selectItems value="#{categoryBean.categories}" var="category" itemValue="#{category.id}" itemLabel="#{category.name}"/>
-        </h:selectOneMenu>
-    </h:form>
+    <ui:repeat value="#{categoryBean.categories}" var="category">
+        <h:outputText value="#{category.name}"/><br/>
+    </ui:repeat>
 
 
     <ui:repeat value="#{categoryBean.books}" var="book" rendered="not empty #{categoryBean.category}">