Fix NULL valued model variable bug

Author: nadhifikbarw

src/Model.php

@@ -1180,7 +1180,7 @@ protected function _relationship($modelName, $relationship, $foreignKey=null, $l
         } else {
             // Original CodeIgniter 3 model loader
             get_instance()->load->model($modelName);
-            $model = $this->$modelName;
+            $model = get_instance()->$modelName;
         }
 
         $libClass = __CLASS__;
@@ -1560,7 +1560,7 @@ public function __get($name)
             } else {
                 // Original CodeIgniter 3 model loader
                 get_instance()->load->model($modelName);
-                $model = $this->$modelName;
+                $model = get_instance()->$modelName;
             }
 
             // Check return type