textarea.gethtml method error when create custom export tool

[Ruby Chang]

Hi, I'm trying to create a custom export tool for my Spotfire Analyst, and I followed the steps here: https://community.spotfire.com/s/article/create-custom-export-tool-tibco-spotfire.

I pastes everything in CustomAddIn.cs and get the error when I run the Debug: 'TextArea' does not contain a definition for 'GetHtml' and no accessible extension method 'GetHtml' accepting a first argument of type 'TextArea' could be found.

I checked the spotfire document, and we do have GetHtml method for TextArea. So I'm not sure how to fix this error. Thanks for your help in advance!