[linux_serial] Exception at PlatformInterface.close

[BaurFilip]

Any call to SerialPortHandle.close() results in an Exception and the underlying fd stays open.

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: type 'Null' is not a subtype of type 'Computer' of 'value'
#0      _LinkedHashMapMixin.[]= (dart:_compact_hash:633)
#1      PlatformInterface.close (package:linux_serial/src/linux_serial.dart:417)
#2      SerialPortHandle.close (package:linux_serial/src/linux_serial.dart:821)