Resolve #3421993 "Primitivetypeconstraintvalidator vs configschemachecker"

Closes #3421993

core/lib/Drupal/Core/Config/Schema/ArrayElement.php

@@ -191,6 +191,8 @@ public function isNullable() {
    */
   public function set($property_name, $value, $notify = TRUE) {
     $this->value[$property_name] = $value;
+    // Changing the value requires re-parsing.
+    unset($this->elements);
     // Config schema elements do not make use of notifications. Thus, we skip
     // notifying parents.
     return $this;