How to obtain iframe html document in NPAPI

I am trying to obtain the document inside a frame.

The following does not fail:

NPN_GetProperty(aInstance, windowObject, NPN_GetStringIdentifier("frames"), &frames)) 

but the following fails, returning a null element:

NPN_Invoke(aInstance, NPVARIANT_TO_OBJECT(frames), NPN_GetStringIdentifier("item"), &index, 1, &currentFrame) 

I've also tried to retrieve all elements with tag IFRAME, but accessing contentWindow or contentDocument property returns a void element.

Are there any other approaches to this?

Finally, I've figured out why contentWindow was returning a void element. Here's the code for obtaining an iframe document :

STRINGZ_TO_NPVARIANT("IFRAME", searchString); 
NPN_Invoke(instanceNPP,NPVARIANT_TO_OBJECT(document), NPN_GetStringIdentifier("getElementsByTagName"), &searchString, 1, &frameCollection);
            
if (!NPN_GetProperty(instanceNPP, NPVARIANT_TO_OBJECT(frameCollection), NPN_GetStringIdentifier("length"), &lenght))
{
    return;
}

for (int i=0; i<NPVARIANT_TO_INT32(lenght); i++)
{
    INT32_TO_NPVARIANT(i, index);
    if (!NPN_Invoke(instanceNPP, NPVARIANT_TO_OBJECT(frameCollection), NPN_GetStringIdentifier("item"), &index, 1, &frameItem))
    {
        continue;
    }
    if (!NPN_GetProperty(instanceNPP, NPVARIANT_TO_OBJECT(frameItem), NPN_GetStringIdentifier("contentWindow"), &contentWindow))
    {
        continue;
    }
    if (!NPN_GetProperty(instanceNPP, NPVARIANT_TO_OBJECT(contentWindow), NPN_GetStringIdentifier("document"), &frameDocument))
    {
        continue;
    }
    //do something with the frame's document
}