Unable to InsertModelSDF into World from model plugin (Bug or Error??)

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I'm working on a ModelPlugin that spawns a new model and adjusts the position of the model based on the other model's position. To do this i create an sdf::SDF from string and spawn it with the function world->InsertModelSDF(modelSDF). The function looks like this:

std::string sensor_pose_as = pose3dToString(this->link_pose);
    sdf::SDF modelSDF;
    modelSDF.SetFromString(
    "<sdf version='1.6'>\
    <model name='Particle_"+this->parent_save_name_+"'>\
    <link name='link_"+this->particle_link_name_+"'>\
      <pose frame=''>"+sensor_pose_as+"</pose>\
      <gravity>0</gravity>\
      <self_collide>0</self_collide>\
      <kinematic>0</kinematic>\
      <visual name='visual_0'>\
        <pose>"+std::to_string(this->p_range)+" 0 0 0 -0 0</pose>\
        <geometry>\
          <box>\
            <size>"+std::to_string(p_size)+" "+std::to_string(p_size)+" 0.001</size>\
          </box>\
        </geometry>\
        <material>\
          <lighting>0</lighting>\
          <script>\
            <uri>file://media/materials/scripts/gazebo.material</uri>\
            <name>Gazebo/Grey</name>\
          </script>\
        </material>\
        <transparency>"+std::to_string(this->p_transparency_)+"</transparency>\
        <cast_shadows>0</cast_shadows>\
      </visual>\
    </link>\
    <static>0</static>\
    </model>\
    </sdf>");
    sdf::ElementPtr model = modelSDF.Root()->GetElement("model");

    for(int i=1; i<=this->num_particles; i++)
    {
            //do other stuff
    }

    ROS_INFO("Insert Model into World");
    this->model_ = model;
    this->world_->InsertModelSDF(modelSDF);
    //this->parent_->GetWorld()->InsertModelString(modelSDF.ToString());

    common::Time::MSleep(1000);
    ROS_INFO("Setting Pose");
    math::Pose p = math::Pose(this->link_pose);
    this->particle_model_ = this->parent_->GetWorld()->GetModel("Particle_"+this->parent_save_name_);

    ROS_INFO("2 %s",this->particle_model_->GetName().c_str());
    this->particle_model_->SetLinkWorldPose(p, this->particle_link_name_);
    ROS_INFO("Particle Model loaded properly");

I used the exact same function in a SensorPlugin and it worked perfectly. When i debugged the programm this

this->particle_model_ = this->parent_->GetWorld()->GetModel("Particle_"+this->parent_save_name_);

always return NULL. Already checked the modelSDF Variable in a extern file and it's okay. Don't know if its a bug since it worked perfectly in my other Plugin.