reverted:
--- b/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
+++ a/core/modules/migrate_drupal_ui/src/Tests/MigrateUpgradeTestBase.php
@@ -131,6 +131,7 @@
     // nice warning.
     $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
     $this->assertText('Resolve the issue below to continue the upgrade.');
+
     $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
     $this->assertResponse(200);
     $this->assertText('Are you sure?');