Angular 5 template drive form issue of dynamic rendering select field as one field changes

I have been facing some issue on angular project on my project. Currently i am using angular 5 and template driven form method. I have created sample here.

The second select field should be filled based on value select from first. the list shows fine but while i change second select field value and the again change first field the second field get extra blank filed . How could i fix this issu?