[CenterType] switch to hidden type with null value when no center is

reachable

src/Bundle/ChillMainBundle/Form/Type/CenterType.php

@@ -76,10 +76,7 @@ class CenterType extends AbstractType
     {
         $nbReachableCenters = count($this->reachableCenters);
 
-        if ($nbReachableCenters === 0) {
-            throw new \RuntimeException("The user is not associated with "
-                    . "any center. Associate user with a center");
-        } elseif ($nbReachableCenters === 1) {
+        if ($nbReachableCenters <= 1) {
             return HiddenType::class;
         } else {
             return EntityType::class;