Symfony 4：“必须实现接口DateTimeInterface”错误

Question

我正在使用简单的管理员捆绑包创建管理员，我真的是Symfony4的新手。

我有一个“创建类别”按钮，当我单击它时，出现此错误：

  

App \ Entity \ Category :: getCreatedAt（）的返回值必须实现接口DateTimeInterface，返回null

代码：

<?php

namespace App\Entity;

use DateInterval;
use DateTimeZone;
use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\Common\Collections\Collection;
use DateTimeInterface;
use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity(repositoryClass="App\Repository\CategoryRepository")
 */
class Category
{

    use TimestampableTrait;

    /**
     * @ORM\Id()
     * @ORM\GeneratedValue()
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @ORM\Column(type="string", length=8)
     */
    private $ref;

    /**
     * @ORM\OneToMany(targetEntity="App\Entity\Product", mappedBy="categoryId")
     */
    private $products;

    /**
     * Category constructor.
     */
    public function __construct()
    {
        $this->products = new ArrayCollection();
    }

    /**
     * @return int|null
     */
    public function getId(): ?int
    {
        return $this->id;
    }

    /**
     * @return string|null
     */
    public function getRef(): ?string
    {
        return $this->ref;
    }

    /**
     * @param string $ref
     * @return $this
     */
    public function setRef(string $ref): self
    {
        $this->ref = $ref;

        return $this;
    }

    /**
     * @return Collection|Product[]
     */
    public function getProducts(): Collection
    {
        return $this->products;
    }

    /**
     * @param Product $product
     * @return $this
     */
    public function addProduct(Product $product): self
    {
        if (!$this->products->contains($product)) {
            $this->products[] = $product;
            $product->setCategoryId($this);
        }

        return $this;
    }

    /**
     * @param Product $product
     * @return $this
     */
    public function removeProduct(Product $product): self
    {
        if ($this->products->contains($product)) {
            $this->products->removeElement($product);
            // set the owning side to null (unless already changed)
            if ($product->getCategoryId() === $this) {
                $product->setCategoryId(null);
            }
        }

        return $this;
    }

    /**
     * (PHP 5 &gt;=5.5.0)<br/>
     * Returns the difference between two DateTime objects
     * @link https://secure.php.net/manual/en/datetime.diff.php
     * @param DateTimeInterface $datetime2 <p>The date to compare to.</p>
     * @param bool $absolute <p>Should the interval be forced to be positive?</p>
     * @return DateInterval
     * The https://secure.php.net/manual/en/class.dateinterval.php DateInterval} object representing the
     * difference between the two dates or <b>FALSE</b> on failure.
     *
     */
    public function diff($datetime2, $absolute = false)
    {
        // TODO: Implement diff() method.
    }

    /**
     * (PHP 5 &gt;=5.5.0)<br/>
     * Returns date formatted according to given format
     * @link https://secure.php.net/manual/en/datetime.format.php
     * @param string $format <p>
     * Format accepted by  {@link https://secure.php.net/manual/en/function.date.php date()}.
     * </p>
     * @return string
     * Returns the formatted date string on success or <b>FALSE</b> on failure.
     *
     */
    public function format($format)
    {
        // TODO: Implement format() method.
    }

    /**
     * (PHP 5 &gt;=5.5.0)<br/>
     * Returns the timezone offset
     * @return int
     * Returns the timezone offset in seconds from UTC on success
     * or <b>FALSE</b> on failure.
     *
     */
    public function getOffset()
    {
        // TODO: Implement getOffset() method.
    }

    /**
     * (PHP 5 &gt;=5.5.0)<br/>
     * Gets the Unix timestamp
     * @return int
     * Returns the Unix timestamp representing the date.
     */
    public function getTimestamp()
    {
        // TODO: Implement getTimestamp() method.
    }

    /**
     * (PHP 5 &gt;=5.5.0)<br/>
     * Return time zone relative to given DateTime
     * @link https://secure.php.net/manual/en/datetime.gettimezone.php
     * @return DateTimeZone
     * Returns a {@link https://secure.php.net/manual/en/class.datetimezone.php DateTimeZone} object on success
     * or <b>FALSE</b> on failure.
     */
    public function getTimezone()
    {
        // TODO: Implement getTimezone() method.
    }

    /**
     * (PHP 5 &gt;=5.5.0)<br/>
     * The __wakeup handler
     * @link https://secure.php.net/manual/en/datetime.wakeup.php
     * @return void Initializes a DateTime object.
     */
    public function __wakeup()
    {
        // TODO: Implement __wakeup() method.
    }
}

我的TimestampableTrait

    <?php

namespace App\Entity;

/**
 * You must add the following comment on all the entities:
 * @ORM\HasLifecycleCallbacks()
 */
trait TimestampableTrait
{

    /**
     * @ORM\ManyToOne(targetEntity="App\Entity\User", inversedBy="createdBy")
     * @ORM\JoinColumn(nullable=true)
     */
    protected $createdBy;

    /**
     * @return User|null
     */
    public function getCreatedBy(): ?User
    {
        return $this->createdBy;
    }

    /**
     * @param User|null $createdBy
     * @return $this
     */
    public function setCreatedBy(?User $createdBy): self
    {
        $this->createdBy = $createdBy;

        return $this;
    }

    /**
     * @var \DateTime
     * @ORM\Column(type="datetime", nullable=true)
     */
    protected $createdAt;

    /**
     * @var \DateTime
     * @ORM\Column(type="datetime", nullable=false)
     */
    protected $updatedAt;

    /**
     * Set the created at value on create
     * @ORM\PrePersist()
     * @param \DateTimeInterface $createdAt
     * @return self
     */
    public function setCreatedAt(\DateTimeInterface $createdAt): self
    {
        $this->createdAt = $createdAt;
        return $this;
    }

    /**
     * Set the updated at value on update
     * @ORM\PrePersist()
     * @ORM\PreUpdate()
     * @param \DateTimeInterface $updatedAt
     * @return self
     */
    public function setUpdatedAt(\DateTimeInterface $updatedAt): self
    {

        $this->updatedAt = $updatedAt;

        return $this;
    }

    /**
     * Get - Created At
     *
     * @return \DateTimeInterface
     */
    public function getCreatedAt(): \DateTimeInterface
    {
        return $this->createdAt;
    }

    /**
     * Get - Updated At
     *
     * @return \DateTimeInterface|null
     */
    public function getUpdatedAt(): \DateTimeInterface
    {
        return $this->updatedAt;
    }

}

我真的不知道问题出在哪里。