I would call customer service to check on your application to see why it's still pending first. You can also send a letter to USCIS asking to withdraw your I-485 because you plan to divorce. Check the receipt notice to see the service center that has it. The letter before "SC" will tell you California, Vermont, Nebraska, etc. National Benefits Center is MSC.
www.gassonlaw.com - Disclaimer: This a general answer to your legal question. Unless you have a signed engagement letter with me, you should not consider this information to be legal advice.
This is an interesting issue and one that should be explored with counsel. There are some ramifications of withdrawing the application as well as continuing with it and then planning on divorce. Both parties should be advise and understand the consequences.