BUG: Exceptions can be null but the get Method requires a string

[erkenes]

The exception property of the TaskExecution class can be null but the get and set methods are requiring a string.

    /**
     * @var string
     * @ORM\Column(nullable = true, type = "text")  //<--- Null is allowed
     */
    protected $exception;

    /**
     * @return string
     */
    public function getException(): string // <--- returns only a string
    {
        return $this->exception;
    }

    /**
     * @param string $exception
     * @return TaskExecution
     */
    public function setException(string $exception): TaskExecution
    {
        $this->exception = $exception;
        return $this;
    }

If you call the method ./flow task:sho <identifier> it is not possible to get a list of the runned tasks: