Issue #3083690 by gxleano: Update trigger error message

Closes #3083690

core/lib/Drupal/Core/Render/Element.php

@@ -95,7 +95,13 @@ public static function children(array &$elements, $sort = FALSE) {
         // Only trigger an error if the value is not null.
         // @see https://www.drupal.org/node/1283892
         elseif (isset($value)) {
-          trigger_error(new FormattableMarkup('"@key" is an invalid render array key', ['@key' => $key]), E_USER_ERROR);
+          trigger_error(new FormattableMarkup(
+            '"@key" is an invalid render array key. Value should be an array but got a @type',
+            [
+              '@key' => $key,
+              '@type' => gettype($value),
+            ]
+          ), E_USER_ERROR);
         }
       }
       $i++;