Can't open file with XmlParts

I save some custom xml parts in excel files with this program :
        public  void SaveContent(string xml )
                TCustomXmlPart xmlPart = new TCustomXmlPart(xml,null);
            catch (Exception e)
                throw new Exception("Erreur lors de la sauvegarde des donn?es dans un document FlexCel :" + "\n" + e.Message);

It works fine but when I open the saved file with this program :
            Xls = new XlsFile();

I get the an XmlException :
Unexpected XML declaration. The XML
declaration must be the first node in the document, and no white space
characters are allowed to appear before it. Line 1, position 41

Here are the first bytes of the xml part (I stripped the null characters) :
<?xml version="1.0" encoding="utf-16"?><Document xmlns:d1="" xmlns:d2="http://sch...
I noticed that BOM was missing, so I tried again with it and got the same exception.
Thanks for your help,

Thanks for reporting this: It is indeed a bug on FlexCel side.
We will be releasing a new version later today that will have this fixed.

Ok, the new version took a little longer than planned due to all changes Microsoft did in .NET core (now it is csproj based, not project.json based).
But the new version is already available on our site.

Thanks a lot