Fix delete problems bug

Author: anhkha2003

judge/views/course.py

@@ -292,7 +292,8 @@ def post(self, request, *args, **kwargs):
             for problem_formset in problem_formsets:
                 problem_formset.save()
                 for obj in problem_formset.deleted_objects:
-                    obj.delete()
+                    if obj.pk is not None:
+                        obj.delete()
             return self.form_valid(formset)
         else:
             return self.form_invalid(formset)