You are using an older version of the API. Please try to use the latest version 21.2 and you won't face this issue. Furthermore, have a look at these migration notes.
Disclosure: I work as a developer evangelist at GroupDocs.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…