fix null pointer if closing an interface which was already shutting down

Author: geeksville

meshtastic/serial_interface.py

@@ -67,9 +67,10 @@ def __init__(self, devPath: Optional[str]=None, debugOut=None, noProto=False, co
 
     def close(self):
         """Close a connection to the device"""
-        self.stream.flush()
-        time.sleep(0.1)
-        self.stream.flush()
-        time.sleep(0.1)
+        if self.stream:  # Stream can be null if we were already closed
+            self.stream.flush()  # FIXME: why are there these  two flushes with 100ms sleeps?  This shouldn't be necessary
+            time.sleep(0.1)
+            self.stream.flush()
+            time.sleep(0.1)
         logging.debug("Closing Serial stream")
         StreamInterface.close(self)