BUG #13989. Fix segfault when deleting proxies.

Fixed segfault when deleting modified proxies. The problem was
that the pqLineEdit fired the focus-lost event as the panel was
being destroyed and the code didn't check for NULL proxy.

Change-Id: I513956bd47f1a5b747ad1c955a177e428766692b
Merge topic '13989_fix_segfault' into next

b96308d BUG #13989. Fix segfault when deleting proxies.