Failed to save prj files-can not open gml file

Hi, I am new in OpenGeoSys learning, and when i use OpenGeoSys Data Exployer to save the project, it always shows that can not open gml file. I tried to remove " " in gml files, but it still does not work. However, when I try to open gml file through “open geometry", gml files can be open.So I wonder what is the causes for this problem. Thank you!