`sensorlib.`get_magnetic_field` and `.get_acceleration` fail with `SensorException`

[aaaaalbert]

Test program, tested on a Google Nexus 6 with Android 5.1.1:

getsensor = dy_import_module("getsensor.r2py")

# Try the sensor calls that require that `start_sensing` 
# has already been called
for sensor_function in [getsensor.get_magnetic_field,
    getsensor.get_acceleration, getsensor.get_light]:
  try:
    log(sensor_function(), "\n")
  except Exception, e:
    log(repr(sensor_function), "failed with", repr(e), "\n")

Problem: get_magnetic_field and get_acceleration request a sensor value before starting the sensor.

(get_light does too, but this doesn't seem to be a problem.)

Log:

========================================
Running program: dylink.r2py
Arguments: ['sensorfails.r2py']
========================================
Using port 63104 to get sensor data.
Something went wrong!
<function get_magnetic_field at 0xb5daecb0> failed with SensorException(u"java.lang.NullPointerException: Null pointer exception during instruction 'monitor-enter v1'",) 
Something went wrong!
<function get_acceleration at 0xb5daec70> failed with SensorException(u"java.lang.NullPointerException: Null pointer exception during instruction 'monitor-enter v1'",) 
None 

Possible solution: Implement a state machine around start_sensing, stop_sensing that ensures that the sensors queried are already set up to return data.