Error while running event MultipleUnitTrainControl::on_init()
MultipleUnitTrainControl/control.lua:69: attempt to index field '?' (a nil value)
stack traceback:
MultipleUnitTrainControl/control.lua:69: in function 'InitEntityMaps'
MultipleUnitTrainControl/control.lua:563: in function <MultipleUnitTrainControl/control.lua:557>

i dont know why

Did you recently install or uninstall another mod? I might not be handling it correctly. If you post your entire factorio-current.log file from when the crash happens that will help.