Problema com getCursos()

quando executo o arquivo de buscar de alunos por curso, o terminal devolve esse erro:

PHP Fatal error: Uncaught TypeError: Return value of Alura\Doctrine\Entity\Student::getCursos() must implement interface Doctrine\Common\Collections\Collection, null returned in /home/oem/php/doctrine-orm/src/Entity/Student.php:85 Stack trace:

#0 /home/oem/php/doctrine-orm/commands/findStudentForClass.php(33): Alura\Doctrine\Entity\Student->getCursos()

#1 {main} thrown in /home/oem/php/doctrine-orm/src/Entity/Student.php on line 85

  <?php

namespace Alura\Doctrine\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;

/**
 * @Entity
 */
class Student {
    /**
     * @ID
     * @GeneratedValue
     * @Column(type="integer")
     */
    private $id;

    /**
     * @Column(type="string")
     */
    private $nome;

    /**
     * @OneToMany(targetEntity="Phone", mappedBy="student", cascade={"remove", "persist"})
     */
    private $phones;

    /**
     * ManyToMany(targetEntity="Curso", mappedBy="student")
     */
    private $cursos;


    public function __construct()
    {
        $this->phones = new ArrayCollection();
        $this->cursos = new ArrayCollection();
    }

    public function getId(): int
    {
        return $this->id;
    }

    public function getNome(): string
    {
        return $this->nome;
    }

    public function setNome(string $nome): self
    {
        $this->nome = $nome;
        return $this;
    }

    public function addPhone(Phone $phone): self     
    {
        $this->phones->add($phone);
        $phone->setStudent($this);//definindo q o telefone pertence ao aluno apos add o telefone.
        return $this;
    }

    public function getPhone(): Collection
    {
        return $this->phones;
    }

    public function addCurso(Curso $curso): self
    {
        if ($this->cursos->contains($curso)) {
            return $this;
        }

        $this->cursos->add($curso);
        $curso->addStudent($this);
        return $this;
    }

    /**
     * @return Curso[]
     */
    public function getCursos(): Collection
    {
        return $this->cursos;
    }
}

<?php

use Alura\Doctrine\Entity\Phone;
use Alura\Doctrine\Entity\Student;
use Alura\Doctrine\Helper\EntityManagerFactory;

require_once __DIR__ . "/../vendor/autoload.php";

$entityManagerFactory = new EntityManagerFactory();
$entityManager = $entityManagerFactory->getEntityManager();


$studentsRepository = $entityManager->getRepository(Student::class);

/**
 * @var Student[] $students
 */
$students = $studentsRepository->findAll();

foreach ($students as $student) {
    $phones = $student
        ->getPhone()
        ->map(function(Phone $phone){
            return $phone->getPhoneNumber();
        })
        ->toArray();

    echo "Id: {$student->getId()}\n";
    echo "Nome: {$student->getNome()}\n";
    echo "Telefone: " . implode(", ", $phones) . "\n";


    $allClass = $student->getCursos();

    foreach  ($allClass as $class) {
        echo "\tID Curso: {$class->getId()}\n";
        echo "\tCurso: {$class->getName()}\n";
        echo "\n";
    }

    echo "\n";
}