Fallback method patch.

Changelog excerpt:
- Precaution against potential future undefined index added to fallback
  method.

Author: Maikuolan

Changelog.md

@@ -55,3 +55,5 @@ __*Why "v3.0.0" instead of "v1.0.0?"*__ Prior to phpMussel v3, the "phpMussel Co
 ### v3.2.2
 
 [2021.06.10; Maikuolan]: Added a flag for successful hits against blacklisted filetypes (needed by the upload handler for a newly added configuration directive). Also did some very minor refactoring.
+
+[2021.09.05; Maikuolan]: Precaution against potential future undefined index added to fallback method.

src/Loader.php

@@ -8,7 +8,7 @@
  * License: GNU/GPLv2
  * @see LICENSE.txt
  *
- * This file: The loader (last modified: 2021.08.25).
+ * This file: The loader (last modified: 2021.09.05).
  */
 
 namespace phpMussel\Core;
@@ -285,11 +285,9 @@ public function __construct(
             $Configuration = $this->readFile($this->AssetsPath . 'config.yml')
         ) {
             $Defaults = [];
-            $this->YAML->process($Configuration, $Defaults, 0, true);
-            if (isset($Defaults)) {
-                $this->fallback($Defaults);
-                $this->ConfigurationDefaults = array_merge_recursive($this->ConfigurationDefaults, $Defaults);
-            }
+            $this->YAML->process($Configuration, $Defaults);
+            $this->fallback($Defaults);
+            $this->ConfigurationDefaults = array_merge_recursive($this->ConfigurationDefaults, $Defaults);
         }
 
         /** Register log paths. */
@@ -541,6 +539,9 @@ public function fallback(array $Fallbacks): void
                 if (isset($Dir)) {
                     unset($Dir);
                 }
+                if (!isset($Cat[$DKey])) {
+                    $Cat[$DKey] = '';
+                }
                 $Dir = &$Cat[$DKey];
                 if (isset($DData['value_preg_filter']) && is_array($DData['value_preg_filter'])) {
                     foreach ($DData['value_preg_filter'] as $FilterKey => $FilterValue) {