When selected items are deleted the Node commands still behave as if the selection exists. Code below shows how to check for a valid selection before using it.

// Execute 
// Called to execute the command 
function Execute(params)
{
    if(!IsValidSelection(Node.Selection())) return;
    System.Trace("selection is good");
}
function IsValidSelection(selection)
{
    if(!selection) return false;
    var reWhiteSpace = /^\s/; // whitespace in first character
    var selectionArray = selection.split(";");
    for(var i = 0; i < selectionArray.length; i++) {
        var selClean = selectionArray[i].replace(reWhiteSpace, "");
        if(!Node.Exists(selClean)) {
            return false;
        }
    }
    return true;
}