Erro é porque o entityManager é sempre nulo.

Spring

https://gist.github.com/anonymous/8bd13cfaecd896be3b2748f6cb103e76

persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
    xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">

    <persistence-unit name="desif" transaction-type="JTA">

        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

         <jta-data-source>java:/desif-ds</jta-data-source>

        <!-- -->
        <class>br.com.netsoft.desif.modelo.desif.ConfiguracaoEntity</class>

        <properties>
            <!-- Configurações específicas do Hibernate -->
            <property name="jboss.entity.validater.jndi.name" value="java:/em" />
            <property name="jboss.entity.manager.factory.jndi.name"
                value="java:/emf/desif" />
            <property name="hibernate.dialect" value="org.hibernate.dialect.PostgreSQLDialect" />
            <property name="hibernate.hbm2ddl.auto" value="update" />
            <property name="hibernate.show_sql" value="true" />
            <property name="hibernate.format_sql" value="true" />
            <property name="show_sql" value="true" />
        </properties>

    </persistence-unit>
</persistence>