When a plug-in enters a bogus file name such as "Untitled.xcd" as URI,
g_file_is_native() returns FALSE. So when saving such file, core code tries to
mount a remote volume and fails (of course), without even opening the file
dialog.
This change is an attempt to detect such cases beforehand and not even try to
save it (just open the file dialog directly; the file name is still useful as
default proposed file name, as this part is actually valid).
Actually remote files will have a valid URI anyway (something with a valid
scheme, such as https:// or whatever other scheme).
As side fix, I am adding a missing space to the error which I got.
473f0b49ef