Unfortunately, it is hard to tell what is going wrong here. To further debug this, please provide a GraphML file which results in this exception.
To do this, you have several options:
a) If the Graph does not contain any personal data, you can just share the gist link with us.
b) Remove any personal data from your graph and either share it with us as gist link or download the file in yEd Live by clicking "Menu" -> "Save" -> "Download". Then you can upload the GraphML to this forum as mentioned in Thomas' comment, or maybe send it directly via mail.