fix exception when checking about:blank response

the current code doesn't make much sense -- if an
HTTP response is not present, it sets 'h' to None...
but then proceeds to do stuff with 'h' assuming it
has some value.

Author: mildsunrise

gp_saml_gui.py

@@ -124,11 +124,15 @@ def on_load_changed(self, webview, event):
         uri = mr.get_uri()
         rs = mr.get_response()
         h = rs.get_http_headers() if rs else None
-        ct = h.get_content_type()
+        ct = h.get_content_type() if h else None
 
         if self.verbose:
             print('[PAGE   ] Finished loading page %s' % uri, file=stderr)
 
+        # if no response or no headers (for e.g. about:blank), skip checking this
+        if not rs or not h:
+            return
+
         # convert to normal dict
         d = {}
         h.foreach(lambda k, v: setitem(d, k.lower(), v))