Why check NPP instance != NULL?

I check:

http://devedge-temp.mozilla.org/library/manuals/2002/plugin/1.0/plugin.pdf

and many sources from:

http://www.google.com/codesearch?hl=ru&lr=&q=lang%3Ac|c\%2B\%2B+return+NPERR_INVALID_INSTANCE_ERROR%3B

They use such pattern:

if (instance==NULL)
     return NPERR_INVALID_INSTANCE_ERROR;

for functions that pass 'NPP instance' arg (like NPP_New, NPP_Destroy,
NPP_GetValue).

Why this need?

Is is possible that browser pass NULL in 'NPP instance' arg?

NOTE: that browser simply can do not invoke func if it know
that NPP object is NULL

NOTE: function call with NULL is not useful useful.

I can not get situation when to NPP_New or NPP_Destroy passed NULL
in Firefox, Opera, Chrome and Safary (most recent stable version
under Windows).

-- 
Best regards!