validation attempt with assert\callback: still doesn't work

src/Bundle/ChillPersonBundle/Entity/AccompanyingPeriod.php

@@ -49,10 +49,6 @@ use UnexpectedValueException;
  *     "accompanying_period": AccompanyingPeriod::class
  * })
  * @Assert\GroupSequenceProvider
- * @Assert\Expression(
- *     "this.isConfidential && this.getUser && this.getStep === 'CONFIRMED'",
- *     message="If the accompanying course is confirmed and confidential, a referrer must remain assigned."
- * )
  */
 class AccompanyingPeriod implements
     TrackCreationInterface,
@@ -860,7 +856,7 @@ class AccompanyingPeriod implements
     }
 
     /**
-     * Validation function.
+     * Validation functions.
      */
     public function isDateConsistent(ExecutionContextInterface $context)
     {
@@ -875,6 +871,21 @@ class AccompanyingPeriod implements
         }
     }
 
+    /**
+     * @Assert\Callback
+     */
+    public function canUserBeNull(ExecutionContextInterface $context)
+    {
+        if ($this->getStep() === self::STEP_CONFIRMED && $this->isConfidential() === true)
+        {
+            if (!$this->getUser()) {
+                $context->buildViolation('User cannot be null for an accompanying period that is confirmed and confidential')
+                    ->atPath('user')
+                    ->addViolation();
+            }
+        }
+    }
+
     public function isEmergency(): bool
     {
         return $this->emergency;