Warning: undefined index 'source'

Seems somewhere schema missing title... however fixing the condition like below would fix the warning.

diff --git a/lib/Drupal/config_translation/Form/ConfigTranslationManageForm.php b/lib/Drupal/config_translation/Form/ConfigTranslationManageForm.php
index 4627a1c..d8ba3f1 100644
--- a/lib/Drupal/config_translation/Form/ConfigTranslationManageForm.php
+++ b/lib/Drupal/config_translation/Form/ConfigTranslationManageForm.php
@@ -169,15 +169,15 @@ class ConfigTranslationManageForm implements FormInterface {
           // check if there is an element which is called title or label, then
           // check if there is an element which contains these words.
           $title = '';
-          if (isset($sub_build['title'])) {
+          if (isset($sub_build['title']['source'])) {
             $title = $sub_build['title']['source']['#markup'];
           }
-          elseif (isset($sub_build['label'])) {
+          elseif (isset($sub_build['label']['source'])) {
             $title = $sub_build['label']['source']['#markup'];
           }
           else {
             foreach (array_keys($sub_build) as $title_key) {
-              if (strpos($title_key, 'title') !== FALSE || strpos($title_key, 'label') !== FALSE) {
+              if (isset($sub_build[$title_key]['source']) && (strpos($title_key, 'title') !== FALSE || strpos($title_key, 'label') !== FALSE)) {
                 $title = $sub_build[$title_key]['source']['#markup'];
                 break;
               }

But not sure it is the good thing to do... will check with @gabor.

Here is the patch...