Java Script Error when creating a new Record

Dear All,

I am new in the Community and JavaScript and have a question regarding a problem I am facing. I am sure the solution is quiet simple but I cant find it I hope you can help me out.

I have created a Customization within Dynamics CRM 2015 and I am trying to get the following accomplished:

I have a JavaScript which is Locking specific fields if the CurrentUserID is NOT equal to the CreatedByID of a record.

I also only want the script to be active when the Form is an Update Form and not on Create.

The script is working fine when I open an existing record. But when I create a new record I get an Error message:

Field: window

Event: onload

Error: undefined 

The script is set to execute OnLoad of the Form.

Here is the code:

function enableCreatedbyEditl() {

var isCreateForm = (Xrm.Page.ui.getFormType() == 1);
var isUpdateForm = (Xrm.Page.ui.getFormType() == 2);
    if (isUpdateForm === true) {

var currUserId = Xrm.Page.context.getUserId();

var currCreatedBy = Xrm.Page.getAttribute("createdby").getValue();

var currCreatedById = currCreatedBy[0].id;

if (currUserId == currCreatedById)

{

Xrm.Page.getAttribute("ownerid").controls.get(0).setDisabled(false);

}

else

{

Xrm.Page.getAttribute("ownerid").controls.get(0).setDisabled(true);

}

}

else if (isCreateFormForm === true) { 
noaction();
}
}

Can anyone let me know how I can get rid of the Error and what my mistake is within the code?

As I said the Script is working the way I need it I just want to get rid of the Error.

Thanks A Lot

Jake