The Mutopia Project has compositions by MutopiaComposer
Wikidata has the property:
Since the template is not very readable: This is a template to link composer biography pages to the Mutopia Project page given in the {{MutopiaComposer|id}} page whose URL ends with id. (To find out id, go to the page http://www.mutopiaproject.org/browse.html#byComposer -- id follows = in the page address (URL) linked to by the composer's name. So Johannes Brahms goes via J. Brahms (1833-1897) to BrahmsJ
Suggestion: for aesthetics at least, please remove any MutopiaComposer links to empty lists, whether or not they were empty when the external link was first made. (There are other reasons to remove links also that are more important - of course.)