MailChimp Forms by Optin Cat missing id

From your question I think you’re using the Display Widget plugin here. I was using Optin Cat and Display Widget together to show specific sidebars on specific posts. The problem was it also displayed “Missing form id” on the topbar of my website. I was entering “123” into the “Comma Separated list of IDs of posts not listed above:” textbox in the widgets admin section. The fix was to change this to “123,”. The plugin was looking for another post id because I didn’t follow each post id with a comma.