The steps for reproducing this bug are:

1) Insert a link from the "Insert link" icon on the rich-text editor and then go to the "Attachment" tab and click on "Browse" to upload an attachment that has not already been attached to that page.
2) Then click on the "OK" button and the exception is generated. Attached the stack trace.
3) Go back to the same upload attachment screen again, the attachment is already present there. A screenshot has been attached.