Issue #3083690 by gxleano, mparker17, jungle, joachim, Kris77: '"@key" is an...

Issue #3083690 by gxleano, mparker17, jungle, joachim, Kris77: '"@key" is an invalid render array key' error in \Drupal\Core\Render\Element is misleading

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++;

core/tests/Drupal/Tests/Core/Render/ElementTest.php

@@ -109,7 +109,7 @@ public function testInvalidChildren() {
       'foo' => 'bar',
     ];
     $this->expectError();
-    $this->expectErrorMessage('"foo" is an invalid render array key');
+    $this->expectErrorMessage('"foo" is an invalid render array key. Value should be an array but got a string');
     Element::children($element);
   }