"java.lang.NullPointerException" error in Java

Good afternoon, I have an error on line 17 of main. I have one "Student" object and another "POO1" object. I wrote the following code:

Aluno aluno = new Aluno ();
Avaliacao poo1 = aluno.getAvaliacao();

To send the notes I wrote this:     // Na main     aluno.setAvaliacao (2.3, 3.1, 5.2, 1.1);

//Na classe Aluno
public void setAvaliacao(double a, double b, double c, double d) {
    Avaliacao x = new Avaliacao (a, b, c, d);
    avaliacao = x;
}

And the Rating constructor:

 public Avaliacao(double nota1, double nota2, double exercicioAula, double trabalhofinal) {
    this.nota1 = nota1;
    this.nota2 = nota2;
    this.exercicioAula = exercicioAula;
    this.trabalhoFinal = trabalhofinal;
}

The get for the notes looks like this:

public Avaliacao getAvaliacao() {
    return avaliacao;
}

And the

private Avaliacao avaliacao;

My question is why is the error appearing when I'm going to print any note on main? Here's how I did the logic to show some note sent:

System.out.println("Nota 1: " + poo1.getTrabalhoFinal()); //Aqui aparece erro
//Erro: Exception in thread "main" java.lang.NullPointerException

If in doubt, follow the class Student:

public class Aluno {
private String nome;
private String matricula;
private Avaliacao avaliacao;

public Aluno() {
}

public String getNome() {
    return nome;
}

public void setNome(String nome) {
    this.nome = nome;
}

public String getMatricula() {
    return matricula;
}

public void setMatricula(String matricula) {
    this.matricula = matricula;
}

public Avaliacao getAvaliacao() {
    return avaliacao;
}

public void setAvaliacao(double a, double b, double c, double d) {
    Avaliacao x = new Avaliacao (a, b, c, d);
    avaliacao = x;
}
}

Follow the Class App:

public class Avaliacao {
private double nota1;
private double nota2;
private double exercicioAula;
private double trabalhoFinal;

public Avaliacao(double nota1, double nota2, double exercicioAula, double trabalhofinal) {
    this.nota1 = nota1;
    this.nota2 = nota2;
    this.exercicioAula = exercicioAula;
    this.trabalhoFinal = trabalhofinal;
}

public double getNota1() {
    return nota1;
}

public void setNota1(double nota1) {
    this.nota1 = nota1;
}

public double getNota2() {
    return nota2;
}

public void setNota2(double nota2) {
    this.nota2 = nota2;
}

public double getExercicioAula() {
    return exercicioAula;
}

public void setExercicioAula(double exercicioAula) {
    this.exercicioAula = exercicioAula;
}

public double getTrabalhoFinal() {
    return trabalhoFinal;
}

public void setTrabalhoFinal(double trabalhoFinal) {
    this.trabalhoFinal = trabalhoFinal;
}
}