diff -u b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
--- b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/CredentialForm.php
@@ -171,9 +171,7 @@
           ':input[name="version"]' => ['value' => '6'],
         ],
       ],
-      '#element_validate' => [
-        [$this, 'validatePaths'],
-      ],
+      '#element_validate' => ['::validatePaths'],
     ];
 
     $form['source']['source_base_path'] = [
@@ -185,9 +183,7 @@
           ':input[name="version"]' => ['value' => '7'],
         ],
       ],
-      '#element_validate' => [
-        [$this, 'validatePaths'],
-      ],
+      '#element_validate' => ['::validatePaths'],
     ];
 
     $form['source']['source_private_file_path'] = [
@@ -200,9 +196,7 @@
           ':input[name="version"]' => ['value' => '7'],
         ],
       ],
-      '#element_validate' => [
-        [$this, 'validatePaths'],
-      ],
+      '#element_validate' => ['::validatePaths'],
     ];
 
     return $form;
@@ -273,10 +267,8 @@
           $this->errors[$element['#name']] = $msg . ' ' . $e->getMessage();
         }
       }
-      else {
-        if (!file_exists($source) || (!is_dir($source)) || (!is_readable($source))) {
-          $this->errors[$element['#name']] = $msg;
-        }
+      else if (!file_exists($source) || (!is_dir($source)) || (!is_readable($source))) {
+        $this->errors[$element['#name']] = $msg;
       }
     }
   }