Did you implement this part?
-for the 2nd action (GoToUrl) use the following for the source name ActionSelectID.RawData and then name is the url parameter that will be appended after the URLMyIDs you specify right after the Type of the action GoToURL
Also your code has an issue, move the line actionResult.RawData = itemIDs; out of the if statement where it is.
Did you try to debug the code? Maybe the error is thrown in your code before hitting the web part.
I can send you my web part and my list template.