I think this must be down to the bug that was fixed upstream in r8304, 2008-11-17. I think that the backend (*telnet_closing():telnet) must have been freed by notify_remote_exit(), leading to telnet->frontend being NULL. (Or at least, I can't see how else telnet->frontend ended up NULL.)
That fix will have first appeared in Ubuntu in putty 0.60+2009-04-05-1. So, the current package should not have this bug.
I think this must be down to the bug that was fixed upstream in r8304, 2008-11-17. I think that the backend (*telnet_ closing( ):telnet) must have been freed by notify_ remote_ exit(), leading to telnet->frontend being NULL. (Or at least, I can't see how else telnet->frontend ended up NULL.)
That fix will have first appeared in Ubuntu in putty 0.60+2009-04-05-1. So, the current package should not have this bug.