In the scenario of RowChecker feature and WebGrid with AddNew feature enabled, you will need to implement the OnRowValidate client side event to resolve this issue.

Here is the snippet of the OnRowValidate client side event, in this snippet the RowChecker column is located in column position 1 of the WebGrid:

function WebGrid1_OnRowValidate(rowObject)
{
  rowObject.childNodes[1].innerHTML = '<input class="DefChkBox" type="checkbox" rowchecker="1" hidefocus="hideFocus"/>';
  return true;
}

This works fine if there is no child table.  However, if I add a child table the error occurs again even with the client rowValidate script.

This issue has been fixed in WebGrid 7 for the next hotfix, the workaround described in the previous post will not be needed using the next hotfix.