Xrm.Page.data Property is NULL when used inside Webresource embedded on a Form

I have a WebResource with html mark-up that draws a html button with an onclick event.  The onclick event calls a javascript function that tries to reference the data property on Xrm.Page.  The html webresource is embedded on a Form in a custom entity.

When I click the html button and debug the script I can see that Xrm.Page.data is NULL. 

I tried calling this same javascript function in the OnLoad event of Form. In this case the Xrm.Page.data property is not null.

 Should i expect the  Xrm.Page.data property to be NULL when embedded in a webreource on a form and populated when used in OnLoad event of the form?