only close output buffer if it is still open

Pjax did alredy close it before ExitException

fixes #8506

framework/CHANGELOG.md

@@ -8,6 +8,7 @@ Yii Framework 2 Change Log
 - Bug #7707: client-side `trim` validator now passes the trimmed value to subsequent validators (nkovacs)
 - Bug #8322: `yii\behaviors\TimestampBehavior::touch()` now throws an exception if owner is new record (klimov-paul)
 - Bug #8451: `yii\i18n\Formatter` did not allow negative unix timestamps as input for date formatting (cebe)
+- Bug #8506: Cleaning of output buffer in `Widget::run()` conflicts with `Pjax` widget which did the cleanup itself (cebe, joester89)
 - Bug: Fixed string comparison in `BaseActiveRecord::unlink()` which may result in wrong comparison result for hash valued primary keys starting with `0e` (cebe)
 - Enh #7169: `yii\widgets\ActiveField` now uses corresponding methods for default parts rendering (klimov-paul)
 - Enh #8070: `yii\console\controllers\MessageController` now sorts created messages, even if there is no new one, while saving to PHP file (klimov-paul)

framework/base/Widget.php

@@ -96,7 +96,10 @@ class Widget extends Component implements ViewContextInterface
             $widget = Yii::createObject($config);
             $out = $widget->run();
         } catch(\Exception $e) {
-            ob_end_clean();
+            // close the output buffer opened above if it has not been closed already
+            if(ob_get_level() > 0) {
+                ob_end_clean();
+            }
             throw $e;
         }