ModemManager crashes if modem resets while being initialized

Environment:

• Ubuntu 20.04 LTS
• ModemManager latest from master branch (also seems to happen with 1.16)
• libqmi latest
• Simcom SIM7600G

When the modem has been in inhibit mode and a modem reset (by AT command without MM) is executed, immediately followed by uninhibit, the modem disappears during modem reinitialization and MM crashes.

Possibly this could also be triggered by a well-timed modem removal.

Call stack:

Thread #1 [ModemManager] 121767 [core: 7] (Suspended : Signal : SIGSEGV:Segmentation fault)	
	mm_gdbus_modem_get_manufacturer() at mm-gdbus-modem.c:26,983 0x7f317a472e7d	
	mm_broadband_modem_create_device_identifier() at mm-broadband-modem.c:12,528 0x5598efb5e6ec	
	modem_load_device_identifier() at mm-broadband-modem-qmi.c:689 0x5598efb8777e	
	interface_initialization_step() at mm-iface-modem.c:5,258 0x5598efb2229a	
	load_equipment_identifier_ready() at mm-iface-modem.c:4,467 0x5598efb24ca0	
	0x7f3179eccf49	
	0x7f3179eccf8d	
	g_main_context_dispatch() at 0x7f3179ce104e	
	0x7f3179ce1400	
	g_main_loop_run() at 0x7f3179ce16f3	
	<...more frames...>	

I've caught this in debugger and the object pointer passed to mm_gdbus_modem_get_manufacturer() is NULL, causing the crash.

Full log here: mm_crash_20210518.log

This seems to reproduce quite easily, so I can test any fix candidates.