PointNormal to PointXYZ transfer problem.

edit

You probably need to resize your target point cloud (mls_cloud). The constructor creates the cloud object, but doesn't know how many points you want to store. You can't just start assigning point data by index until you first allocate space to store them.

Try this:

pcl::PointCloud<pcl::PointXYZ>::Ptr mls_cloud (new pcl::PointCloud<pcl::PointXYZ>); 
mls_cloud->resize(mls_points.size());

for (size_t i = 0; i < mls_points.points.size(); ++i) 
{ 
    mls_cloud->points[i].x=mls_points.points[i].x; //error 
    mls_cloud->points[i].y=mls_points.points[i].y; //error 
    mls_cloud->points[i].z=mls_points.points[i].z; //error 
}

Alternatively, you can dynamically build the cloud if you don't know ahead-of-time how many points it will contain:

pcl::PointCloud<pcl::PointXYZ>::Ptr mls_cloud (new pcl::PointCloud<pcl::PointXYZ>); 

for (size_t i = 0; i < mls_points.points.size(); ++i) 
{
    const pcl::PointNormal &mls_pt = mls_points.points[i];
    pcl::PointXYZ pt(mls_pt.x, mls_pt.y, mls_pt.z);
    mls_cloud.push_back(pt);
}

Or, even better, don't re-create the wheel, and use the built-in PCL copyPointCloud function to convert cloud types:

pcl::PointCloud<pcl::PointXYZ>::Ptr mls_cloud (new pcl::PointCloud<pcl::PointXYZ>);
copyPointCloud(cloud_xyz, cloud_xyzrgb);