is_non_critical MCU: QUERY_ENDSTOPS causes error shutdown

[mk-maddin]

The problem:

• when a disconnected MCU provides an endstop pin QUERY_ENDSTOPS causes error shutdown

Steps to reproduce:

• some MCU is defined as is_non_critical: true
• the MCU has some endstop connect (can be some axis endstop, some probe, some happyhare filament endstop etc. - basically anything using the internal endstop object)
• the MCU is disconnected
• QUERY_ENDSTOPS command is executed via console or via mainsail web interface

The behavior:

• Kalico error shutdown:

• following three errors can be found in klippy.log

Clock not yet synchronized, clock is untrustworthy
File "/home/pi/klipper/klippy/reactor.py", line 402, in _dispatch_loop
    timeout = self._check_timers(eventtime, busy)
File "/home/pi/klipper/klippy/reactor.py", line 193, in _check_timers
    t.waketime = waketime = t.callback(eventtime)
File "/home/pi/klipper/klippy/reactor.py", line 66, in invoke
    res = self.callback(eventtime)
File "/home/pi/klipper/klippy/webhooks.py", line 303, in <lambda>
    lambda e, s=self, wr=web_request: s._process_request(wr)
File "/home/pi/klipper/klippy/webhooks.py", line 309, in _process_request
    func(web_request)
File "/home/pi/klipper/klippy/webhooks.py", line 527, in _handle_script
    self.gcode.run_script(web_request.get_str("script"))
File "/home/pi/klipper/klippy/gcode.py", line 345, in run_script
    self._process_commands(script.split("\n"), need_ack=False)
File "/home/pi/klipper/klippy/gcode.py", line 318, in _process_commands
    handler(gcmd)
File "/home/pi/klipper/klippy/gcode.py", line 227, in func
    return origfunc(self._get_extended_params(params))
File "/home/pi/klipper/klippy/extras/query_endstops.py", line 54, in cmd_QUERY_ENDSTOPS
    (name, mcu_endstop.query_endstop(print_time))
File "/home/pi/klipper/klippy/mcu.py", line 478, in query_endstop
    clock = self._mcu.print_time_to_clock(print_time)
File "/home/pi/klipper/klippy/mcu.py", line 1375, in print_time_to_clock
    return self._clocksync.print_time_to_clock(print_time)
File "/home/pi/klipper/klippy/clocksync.py", line 248, in print_time_to_clock
    for line in traceback.format_stack():
Internal error on command:"QUERY_ENDSTOPS"
Traceback (most recent call last):
  File "/home/pi/klipper/klippy/gcode.py", line 318, in _process_commands
    handler(gcmd)
    ~~~~~~~^^^^^^
  File "/home/pi/klipper/klippy/gcode.py", line 227, in func
    return origfunc(self._get_extended_params(params))
  File "/home/pi/klipper/klippy/extras/query_endstops.py", line 54, in cmd_QUERY_ENDSTOPS
    (name, mcu_endstop.query_endstop(print_time))
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/home/pi/klipper/klippy/mcu.py", line 481, in query_endstop
    params = self._query_cmd.send([self._oid], minclock=clock)
             ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'send'
Transition to shutdown state: Internal error on command:"QUERY_ENDSTOPS"

Internal Error on WebRequest: gcode/script
Traceback (most recent call last):
  File "/home/pi/klipper/klippy/webhooks.py", line 309, in _process_request
    func(web_request)
    ~~~~^^^^^^^^^^^^^
  File "/home/pi/klipper/klippy/webhooks.py", line 527, in _handle_script
    self.gcode.run_script(web_request.get_str("script"))
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/pi/klipper/klippy/gcode.py", line 345, in run_script
    self._process_commands(script.split("\n"), need_ack=False)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/pi/klipper/klippy/gcode.py", line 318, in _process_commands
    handler(gcmd)
    ~~~~~~~^^^^^^
  File "/home/pi/klipper/klippy/gcode.py", line 227, in func
    return origfunc(self._get_extended_params(params))
  File "/home/pi/klipper/klippy/extras/query_endstops.py", line 54, in cmd_QUERY_ENDSTOPS
    (name, mcu_endstop.query_endstop(print_time))
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/home/pi/klipper/klippy/mcu.py", line 481, in query_endstop
    params = self._query_cmd.send([self._oid], minclock=clock)
             ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'send'

Internal error on command:"QUERY_ENDSTOPS"
Once the underlying issue is corrected, use the
"FIRMWARE_RESTART" command to reset the firmware, reload the
config, and restart the host software.
Printer is shutdown

Script running error
Traceback (most recent call last):
  File "/home/pi/klipper/klippy/extras/delayed_gcode.py", line 51, in _gcode_timer_event
    self.gcode.run_script(self.timer_gcode.render())
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/pi/klipper/klippy/gcode.py", line 345, in run_script
    self._process_commands(script.split("\n"), need_ack=False)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/pi/klipper/klippy/gcode.py", line 318, in _process_commands
    handler(gcmd)
    ~~~~~~~^^^^^^
  File "/home/pi/klipper/klippy/gcode.py", line 406, in cmd_default
    raise gcmd.error(self.printer.get_state_message()[0])
klippy.gcode.CommandError: Internal error on command:"QUERY_ENDSTOPS"
Once the underlying issue is corrected, use the
"FIRMWARE_RESTART" command to reset the firmware, reload the
config, and restart the host software.
Printer is shutdown

It might be possible that this is related to #900 - but I have not enough code insights to evaluate this