[ENSWEB-2460] _create_idhistory_tree returns undef for an export request
[ENSWEB-2460] _create_idhistory_tree returns undef for an export request

Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode