[Yii2] Check if Yii::$app is set in _after before using it

Fixes http://stackoverflow.com/questions/39810706/using-fixtures-in-functional-test-for-yii2-application

Author: Naktibalda

src/Codeception/Module/Yii2.php

@@ -194,7 +194,7 @@ public function _after(\Codeception\TestInterface $test)
 
         $this->client->resetPersistentVars();
 
-        if (\Yii::$app->has('session', true)) {
+        if (isset(\Yii::$app) && \Yii::$app->has('session', true)) {
             \Yii::$app->session->close();
         }
         parent::_after($test);