I having a little difficulty trying to reproduce this issue. If you can please post back and include specific steps that I can take to get the result you are getting. With the same steps and results I will better be able to help troubleshoot the problem.
As for the error you are getting when you edit in wizard when selecting the param I'm not able to get this to occur on my end. I have tried it a few different ways and each time I'm able to edit the chart in the wizard without issue. Are there any other steps you are taking when you get this error? Does this error occur for you each and every time you attempt to edit with the wizard when you have the code selected?