Use selected_SequenceSet instead of sequence_sets_cached
requested to merge github/fork/thibauthourlier/hotfix/dna_disappearing into feature/ensembl_friendly
Created by: thibauthourlier
selected_SequenceSet should be set when we have selected the region to work on so it will return the correct coord system name and version whereas using the first element of sequence_sets_cached may return the wrong corrd system name