Avoid this error:

NetworkManager[25181]: <warn> (cdc-wdm1): Failed to connect 'T-Mobile Internet': Connection requested IPv4 but IPv4 is unsuported by the modem.
NetworkManager[25181]: <info> (cdc-wdm1): device state change: prepare -> failed (reason 'modem-init-failed') [40 120 28]

** (NetworkManager:25181): CRITICAL **: mm_modem_simple_disconnect: assertion 'MM_IS_MODEM_SIMPLE (self)' failed

self->priv->simple_iface is only valid if stage1/prepare actually
completes, so don't try to access it if stage1/prepare failed.