PowerMockito static class return NullPointerException

Question

I have a problem mocking the static class TransportClient. When I run the tests, the first one to be run works fine, while the second throws a NullPointerException. This does not depend on which one is the first run.

I discovered the reason. The second time client.admin returns null instead the mock class adminClientMock:
1° Test run:

System.out.println(client) -> mock TransportClient
System.out.println(client.admin()) -> mock AdminClient

2° Test run:

System.out.println(client) -> mock TransportClient
System.out.println(client.admin()) -> null

Any idea? I know it is not really clear but the classes call are a lot. Anyway I will appreciate any kind of help.

    public class ES{
    private final Client client;
    private ES() {
    client = TransportClient.builder().settings(settings).build().addTransportAddress(new InetSocketTransportAddress(esAddress));
    }

    public void method{
    client.admin().indices().preparePutTemplate(template.getString("name"))
                                        .setSource(mapping)
                                        .execute().actionGet();
    }

    @RunWith(PowerMockRunner.class)
    @PrepareForTest({Settings.class, Configuration.class, TransportClient.class, InetSocketTransportAddress.class, ES.class})
    public class ESTest{

     private initMocks(){
            // mock TransportClient class
            PowerMockito.mockStatic(TransportClient.class);
            TransportClient transportClientMock = PowerMockito.mock(TransportClient.class);
            TransportClient.Builder mockTransportBuilder = PowerMockito.mock(TransportClient.Builder.class);
            when(TransportClient.builder()).thenReturn(mockTransportBuilder);
            when(mockTransportBuilder.settings(mockSettings)).thenReturn(mockTransportBuilder);
            when(mockTransportBuilder.build()).thenReturn(transportClientMock);
            Mockito.when(transportClientMock.addTransportAddress(any(InetSocketTransportAddress.class))).thenReturn(transportClientMock);


            AdminClient adminClientMock = mock(AdminClient.class);
            PowerMockito.when(transportClientMock.admin()).thenReturn(adminClientMock);
    }

 @Test
    public void Test1(){
        ES clientES = ES.getInstance();
.....
}

 @Test
    public void Test2(){
        ES clientES = ES.getInstance();
.....
}

}