Fix: Delete objects even when no transport is available

The issue is that objects that get deleted will not be
removed from the internal lists.
This patch checks for destroyed signals in cases when
no transport is available (no connection established).

cherry picked from commit 037c67962de3d74669c55a9db456ffd38a28480f

Change-Id: I24e345dcbfe88e15031d288eb65abb2c3066d4c0
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>