Can't export my translated file. Get error message: "Failed to save target content: Value cannot be null.Parameter name: fileName"

Finished translating my first job with Trados Studio and tried to export the finished file using "Save Target As" and got the error message "Failed to save target content: Value cannot be null.Parameter name: fileName". I've removed some messages that had appeared in the Edit screen, but it still won't export the file. What's wrong?