DITA+FM ERROR: Specified FileOpenClient not installed.: XXXX.xml - Unable to resolve xref

I created a ditamap.

When I open an xml file which contains an xref, the console displays this message:

DITA+FM ERROR: Specified FileOpenClient not installed.: XXXX.xml - Unable to resolve xref.

But the reference still works if I press ctrl + alt and click.

I'm just curious about what it exactly means as I found nothing useful about that from the Internet.

Does anyone know?