"session is null" exception in Firefox

[bug]

session is null
? in Orbited.js@891(Object { name="data"})Orbited.js (Zeile 900)
? in stomp.js@127(Object { name="data"})stomp.js (Zeile 129)
? in stomp.js@431(Object { name="type"}, Object { name="headers"}, Object { name="body"})stomp.js (Zeile 457)
? in stomp.js@505(Object { name="destination"}, Object { name="extraHeaders"})stomp.js (Zeile 506)
? in 27@55()27 (Zeile 56)
onunload(Object { name="event"})

This happens right at the beginning when Orbited.js is loaded. It works correctly afterwards.