error on line 1 at column 10: XML declaration allowed only at the start of the document

Anyone found solution of this issue? I have the same problem and can't find what is causing it

Ok, I actually just resolved my issue: it was caused by empty line in my MYTHEME.theme file. But it might be also another file in theme. It's worth to verify if it's somewhere in the theme by switching to default theme and checking if error still occurs. I'm leaving it here, as it may help somebody.