You need to show your code for specific solution. But this exception normally comes generated when an invalid value is passed to a method argument. I believe, there is no relation with having a html editor in your aspx with this error. check your methods a null or empty is being passed where it should not be.
Try to do some defensive programming to avoid these type of errors. It is always better to check for nulls where appropriate.
You may look into "Design by Contract
" concept which can be a safe guard for this type of issues.