remove-machine: Add a missing null check
remove-machine: Add a missing null check

Before accessing the parsed Xml.URI object, make sure it's not null.
This code is called multiple times while user is editting the URL and if
it gets called at a time when entered URL is not valid, we'll get a
crash if we don't have this check.

vbenes/gnome-boxes,vbenes/gnome-boxes