bind a select with entity and it could be null

Catchable Fatal Error: Argument 1 passed to MyProject\EntityBundle\Entity\Requirements::setReplacedEmployee() must be an instance of MyProject\EntityBundle\Entity\Employee, null given, called in /var/www/MyProject/vendor/symfony/src/Symfony/Component/Form/Util/PropertyPath.php on line 347 and defined in /var/www/MyProject/src/MyProject/EntityBundle/Entity/Requirements.php line 384

Should do this:

I can't fully understand your question but, if you want to allow null values for the relation with Employee you should first edit the mapping (this maybe not necessary as JoinColumn should allow null values by default):
 * @var replacedEmployee
 * @ORM\ManyToOne(cascade={"persist"},targetEntity="Employee")
 * @ORM\JoinColumns({
 *   @ORM\JoinColumn(
 *     name="replaced_employee_id",referencedColumnName="id",onDelete="CASCADE",
 *     nullable=true
 *   )
 * })
private $replacedEmployee;
After generating setters/getters Doctrine2 (if i remember correctly, starting from 2.2.1) should generate:
 * Set replacedEmployee
 * @param MyProject\EntityBundle\Entity\Employee $replacedEmployee
public function setReplacedEmployee(Employee $replacedEmployee = null)
    $this->replacedEmployee = $replacedEmployee;
Note that argument is optional (has null default value). Hope this helps.