NullPointerException when running JUnit test with TestEntityManager in Spring Boot

Error Description: When running the test findByEmail in the ClientRepositoryTest class, a NullPointerException occurs with the message "Cannot invoke org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager.persist(Object) because com.saajf.manager.eewaapro.ClientRepositoryTest.entityManager is null."

Code Snippet:

package com.saajf.manager.eewaapro;

import static org.assertj.core.api.Assertions.assertThat;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
import org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager;

import com.saajf.manager.eewaapro.domain.entities.Client;
import com.saajf.manager.eewaapro.infrastructure.repositories.ClientRepository;

@DataJpaTest
class ClientRepositoryTest {

    @Autowired
    private ClientRepository clientRepository;

    private static TestEntityManager entityManager;

    @Test
    void findByEmail() {
        Client client = new Client();
        client.setId(1L);
        client.setCompanyAddress1("cr 54 # 143a - 90");
        client.setCompanyName("cualquier cosa");
        client.setCompanyType("Software");
        client.setCompanyPhone1("3165476589");
        client.setEmail("[email protected]");
        client.setIdentification("123456789");

        // Guardar el cliente utilizando el entityManager persistente
        Client savedClient = entityManager.persist(client);

        // Limpiar el entityManager para asegurar que se recargue el cliente desde la
        // base de datos
        entityManager.clear();

        // Realizar la búsqueda del cliente por email
        Client foundClient = clientRepository.findByEmail(savedClient.getEmail());

        assertThat(foundClient).isEqualTo(savedClient);
    }
}

pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <!-- Parent -->
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.7.11</version>
        <relativePath/>
    </parent>
    <groupId>com.atukaa.eewaapro.manager</groupId>
    <artifactId>eewaapro-manager</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>eewaapro-manager</name>
    <description>Manager component for the Atukaa Eewaapro Service</description>
    <properties>
        <java.version>17</java.version>
    </properties>
    <!-- Dependencies -->
    <!-- Add your dependencies here -->
    ...
    <!-- Build -->
    <!-- Add your build configuration here -->
    ...
</project>

Stack Overflow Question: Title: NullPointerException when running JUnit test with TestEntityManager

I'm experiencing a NullPointerException when running a JUnit test with TestEntityManager. The error message states: "Cannot invoke org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager.persist(Object) because com.saajf.manager.eewaapro.ClientRepositoryTest.entityManager is null."

contextLoader = 'org.springframework.boot.test.context.SpringBootContextLoader', parent = [null]], attributes = map['org.springframework.test.context.event.ApplicationEventsTestExecutionListener.recordApplicationEvents' -> false]] [[1;31mERROR[m] [1;31mTests [0;1mrun: [0;1m1[m, Failures: 0, [1;31mErrors: [0;1;31m1[m, Skipped: 0, Time elapsed: 4.892 s[1;31m <<< FAILURE![m - in com.saajf.manager.eewaapro.[1mClientRepositoryTest[m [[1;31mERROR[m] findByEmail Time elapsed: 0.239 s <<< ERROR! java.lang.NullPointerException: Cannot invoke "org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager.persist(Object)" because "com.saajf.manager.eewaapro.ClientRepositoryTest.entityManager" is null at com.saajf.manager.eewaapro.ClientRepositoryTest.findByEmail(ClientRepositoryTest.java:33)

2023-05-15 23:24:32.471 INFO 13068 --- [ionShutdownHook] j.LocalContainerEntityManagerFactoryBean : Closing JPA EntityManagerFactory for persistence unit 'default'