/admin/content/migrate_source_ui leads to /core/install.php

/admin/config/content/migrate_source_ui works, I'm double checking if there is some kind of redirect.

Ok, so it is a code issue, possibly related to https://www.drupal.org/project/drupal/issues/2974321 →

- The form is called, but the redirect happens during the buildForm method in this loop:

foreach ($this->definitions as $definition) {
      $migrationInstance = $this->pluginManagerMigration->createStubMigration($definition);
      if ($migrationInstance->getSourcePlugin() instanceof CSV || $migrationInstance->getSourcePlugin() instanceof Json || $migrationInstance->getSourcePlugin() instanceof Xml) {
        $id = $definition['id'];
        $options[$id] = $this->t('%id (supports %file_type)', [
          '%id' => $definition['label'] ?? $id,
          '%file_type' => $this->getFileExtensionSupported($migrationInstance),
        ]);
      }
    }

The second of our 6 migrations is already bringing it down - the used plugin is `custom_sql_query` - I guess $this->pluginManagerMigration->createStubMigration($definition) is doing something very ressource intentsive under the hood for this.

I propose do delay this call as late as possible.

This patch works for me - a little drawback is, that I had to hardcode the plugin id, but it takes too much time now, to introduce a static getId method to the migrate universe.