error: ‘this’ argument discards qualifiers [-fpermissive]

edit

This is my code for the Adaptive Monte Carlo localization(AMCL) algorithm, a plugin using for ign gazebo(citadel) and ROS(melodic).

void amcl::Configure(const Entity &_entity, const std::shared_ptr<const sdf::element=""> &_sdf, EntityComponentManager &_ecm, EventManager &_eventMgr)

{ this->dataPtr->model = Model(_entity); if (!this->dataPtr->model.Valid(_ecm)) {

ignerr << "amcl plugin should be attached to a model entity. "
       << "Failed to initialize." << std::endl;
return;
}

// Obtain parameters from xacro file

if(_sdf->HasElement("topicRate"))
{
    rate_ = _sdf->GetElement("topicRate")->Get<double>(); #############ERROR IN THIS LINE 

}
else
{
  rate_ = 10.0;
}


if(_sdf->HasElement("fixedFrame"))

{
    fixed_frame_ = _sdf->GetElement("fixedFrame")->Get<std::string>(); ########ERROR IN THIS LINE 

}
else
{
    fixed_frame_ = "map";
}

if(_sdf->HasElement("robotFrame"))

{
    robot_frame_ = _sdf->GetElement("robotFrame")->Get<std::string>(); #####ERROR IN THIS LINE

}
else
{
    robot_frame_ = "base_link";
}

Error (as shown in the terminal)

error: passing ‘std::__shared_ptr_access<const sdf::v9::element,="" (__gnu_cxx::lock_policy)2,="" false,="" false="">::element_type’ {aka ‘const sdf::v9::Element’} as ‘this’ argument discards qualifiers [-fpermissive] rate = _sdf->GetElement("topicRate")->Get();

I have marked the lines in the code which throw this error.

Complete code can be found here with the header file (Amcl.hh)

Can anybody suggest a solution?