When the user launches Settings with a panel passed
as argument, the following things happen:
1. The Wi-Fi panel starts loading.
2. The command line arguments are passed and the given
panel is activated.
3. The Wi-Fi panel cancels the loading routine, and
rfkill_proxy_acquired_cb() is called with the GError
set as G_IO_ERROR_CANCELLED.
4. Crash in rfkill_proxy_acquired_cb().
The crash is caused because, when rfkill_proxy_acquired_cb()
is called, the CcWifiPanel instance isn't valid anymore. And
yet, the code tries to cast 'gpointer user_data' to a
CcWifiPanel pointer.
Fix that by only trying to cast anything after parsing the
GError set by the callback.
fcf4e44ebd