Hi Giacinto,
>     0  0x0000563b1842b735 in call_watches
> (atom=atom(a)entry=0x563b1a3f07d0,
> cond=cond(a)entry=OFONO_ATOM_WATCH_CONDITION_UNREGISTERED) at
> src/modem.c:265
>     1  0x0000563b1842c32e in __ofono_atom_unregister
> (atom=atom(a)entry=0x563b1a3f07d0) at src/modem.c:299
>     2  0x0000563b1842c3ff in __ofono_atom_unregister
> (atom=0x563b1a3f07d0) at src/modem.c:296
>     3  flush_atoms (modem=0x563b1a3f1f50,
> new_state=MODEM_STATE_POWER_OFF) at src/modem.c:448
>     4  modem_change_state (modem=0x563b1a3f1f50,
> new_state=MODEM_STATE_POWER_OFF) at src/modem.c:529
>     5  0x0000563b1842c577 in set_powered
> (modem=modem(a)entry=0x563b1a3f1f50, powered=powered(a)entry=0) at
> src/modem.c:915
>     6  0x0000563b1842c863 in modem_unregister
> (modem=modem(a)entry=0x563b1a3f1f50) at src/modem.c:2111
>     7  0x0000563b1842e6a4 in ofono_modem_remove (modem=0x563b1a3f1f50)
> at src/modem.c:2177
>     8  0x0000563b183b05d3 in destroy_modem (data=0x563b1a3e50a0) at
> plugins/udevng.c:1408
>     9  0x00007f42b4bdd091 in ?? () from
> /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
>     10 0x0000563b183b561a in remove_device (device=0x563b1a401630) at
> plugins/udevng.c:1468
>     11 udev_event (channel=<optimized out>, cond=<optimized out>,
> user_data=<optimized out>) at plugins/udevng.c:1991
>     12 0x00007f42b4bef0f5 in g_main_context_dispatch () from
> /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
>     13 0x00007f42b4bef4c0 in ?? () from
> /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
>     14 0x00007f42b4bef7d2 in g_main_loop_run () from
> /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
>     15 0x0000563b183b0397 in main (argc=<optimized out>,
> argv=<optimized out>) at src/main.c:294
> 
> ?

Yes, much better.  So now you can explain to me how you're triggering this.

The only way modem->atom_watches is NULL is if ofono_modem_register 
failed.  So are you trying to use a modem object without registering it 
properly?

Regards,
-Denis