Fix #3, thanks @TemaSM and @AEK-BKF

Author: youchenlee

src/Setting/Setting.php

@@ -34,6 +34,9 @@ public function get($key, $default_value = null)
             }
         }
         $this->resetLang();
+        if (is_null($setting)) {
+            $setting = $default_value;
+        }
         return $setting;
     }