Issue #2951684 by alexpott, quietone: Undefined index ReviewForm->buildForm() line 240

core/modules/migrate_drupal_ui/src/Form/ReviewForm.php

@@ -237,7 +237,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     // Add source_module and destination_module for modules that do not need an
     // upgrade path and are enabled on the source site.
     foreach ($this->noUpgradePaths[$version] as $extension) {
-      if ($system_data['module'][$extension]['status']) {
+      if (isset($system_data['module'][$extension]) && $system_data['module'][$extension]['status']) {
         $table_data[$extension]['core'][$extension] = $extension;
       }
     }

core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeReviewPageTestBase.php

@@ -112,6 +112,28 @@ public function testMigrateUpgradeReviewPage() {
     $available_paths = $this->getAvailablePaths();
     $missing_paths = $this->getMissingPaths();
     $this->assertUpgradePaths($session, $available_paths, $missing_paths);
+
+    // Check there are no errors when a module in noUpgradePaths is not in the
+    // source system tables. Test with a module that is listed in noUpgradePaths
+    // for both Drupal 6 and Drupal 7.
+    // @see \Drupal\migrate_drupal_ui\Form\ReviewForm::$noUpgradePaths
+    $module = 'help';
+    $query = $this->sourceDatabase->delete('system');
+    $query->condition('type', 'module');
+    $query->condition('name', $module);
+    $query->execute();
+
+    // Start the upgrade process.
+    $this->drupalGet('/upgrade');
+    $this->drupalPostForm(NULL, [], t('Continue'));
+    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data. Continue anyway.'));
+
+    // Test the upgrade paths.
+    $available_paths = $this->getAvailablePaths();
+    $available_paths = array_diff($available_paths, [$module]);
+    $missing_paths = $this->getMissingPaths();
+    $this->assertUpgradePaths($session, $available_paths, $missing_paths);
   }
 
   /**