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Modem DBus interface unusable if SIM card not inserted

Problem:

$ ./cli/mmcli -L
    /org/freedesktop/ModemManager1/Modem/0 [Sierra Wireless, Incorporated] MC7455
$ ./cli/mmcli -d -m 0
error: couldn't disable the modem: 'GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: Method Enable is not implemented on interface org.freedesktop.ModemManager1.Modem'

Looking at the code, I worked out that the interface was not being constructed as the SIM card was not inserted correctly. I would have been much better to connect up the interface, and return something like org.freedesktop.ModemManager1.Modem.SimCardNotInserted -- although for something like Enable() I don't think the missing SIM card should have affected my ability to power down the modem...

In mm-iface-modem.c:4988 the problem is treated as fatal, and Handle method invocations never gets run.

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