Unable to get property 'then' of undefined or null reference

Hello everyone,

I have an error happens at Xrm.Page.data.save(saveOptions).then(successCallback, errorCallback) function.

My situation is like following: 

• I have a plugin that checks for some business rule at (Service Appointment) form.
• Business Rule is to prevent any user of creating new (Service Appointment) if he is already have another (Scheduled) one.
• I created the plugin as (PreCreate) to run before save new (Service Appointment)
• Plugin working normally, and throw (InvalidPluginExecutionException) when business rule is violated
• Now I wrote a JavaScript to handle (InvalidPluginExecutionException) to show error message as a Form notification using ( Xrm.Page.ui.setFormNotification)
• I attached that new JavaScript to (Service Appointment) Form to be fired (OnSave) event.
• Following code clarify the JavaScript code

function StartSave() {
    //========================================================
    //Clear old notifications
    Xrm.Page.ui.clearFormNotification();

    //Save Form data
    Xrm.Page.data.save().then(SuccessCallback, ErrorCallback);
}

• Now error happens at (Xrm.Page.data.save().then). Error is (Unable to get property 'then' of undefined or null reference).

I don't know why this error happens and how to know its root cause. Is OnSave event is not correct?? 

I used following links samples to mimic its behavior:

• http://missdynamicscrm.blogspot.sg/2015/06/field-required-mandatory-crm-ribbon-onsave-action-javascript.html
• http://missdynamicscrm.blogspot.com/2015/06/pass-plugin-context-message-c-exception-to-crm-javascript-ribbon.html
• http://missdynamicscrm.blogspot.com/2015/06/convert-crm-plugin-message-to-crm-alert-form-notification-javascript-from-plugin-csharp.html?m=1

I use CRM 2015 on Premise.

Thanks in advance.

Kind Regards,

Wa'el Mohsen