Code: bad_function_call after clicking "Disconnect"

After fixing the NaN bug in 5dd55a5f another one is on the list to fix is the dreaded bad_function_call crash when disconnecting from serial interface. Probably object ceases to exist before callback thread is stopped so it tries to call the function of a non-existent object. We could rectify that by setting callback to no function or something so that last call gets ignored. This of course requires some looking into, but this is my best guess about what is happening under the hood.