Fix setting of group

Author: paulbalandan

system/Database/MigrationRunner.php

@@ -141,10 +141,7 @@ public function __construct(MigrationsConfig $config, $db = null)
         // Default name space is the app namespace
         $this->namespace = APP_NAMESPACE;
 
-        // get default database group
-        $config      = config(Database::class);
-        $this->group = $config->defaultGroup;
-        unset($config);
+        $this->group = is_string($db) ? $db : config(Database::class)->defaultGroup;
 
         // If no db connection passed in, use
         // default database group.
@@ -841,7 +838,7 @@ protected function migrate($direction, $migration): bool
 
         /** @var Migration $instance */
         $instance = new $class(Database::forge($this->db));
-        $group    = $instance->getDBGroup() ?? config(Database::class)->defaultGroup;
+        $group    = $instance->getDBGroup() ?? $this->group;
 
         if (ENVIRONMENT !== 'testing' && $group === 'tests' && $this->groupFilter !== 'tests') {
             // @codeCoverageIgnoreStart
@@ -857,8 +854,6 @@ protected function migrate($direction, $migration): bool
             return true;
         }
 
-        $this->setGroup($group);
-
         if (! is_callable([$instance, $direction])) {
             $message = sprintf(lang('Migrations.missingMethod'), $direction);