Fix generate data for flatview in gradebook - refs #8218

main/gradebook/lib/flatview_data_generator.class.php

@@ -355,7 +355,10 @@ class FlatViewDataGenerator
 
         $parent_id = $this->category->get_parent_id();
 
-        if ($parent_id == 0 or $this->params['only_subcat'] == $this->category->get_id()) {
+        if (
+            $parent_id == 0 ||
+            (isset($this->params['only_subcat']) && $this->params['only_subcat'] == $this->category->get_id())
+        ) {
             $main_weight  = $this->category->get_weight();
             $grade_model_id = $this->category->get_grade_model_id();
         } else {