Comment 4 for bug 696761

Tests prove its possible that when HubFrame::preClosing is called the password dialog is beeing / already shown. When the dialog closed then the Client object is already destroyed in the hubframe destructor, that's why it crashes.
If its possible to check somehow that the window is beeing closed (even before preClosing called) then the dialog can be prevented to show using that way (beyond my competence)
If not, both of the two attached patch prevents the crash, choose the better looking one :)