ArcGIS Runtime SDF for Android UniqueValueRenderer does not work when value is null

[LTHDAFS]

I set a two field UniqueValueRenderer for featurelayer,  the first field has a vaule, the Second field may has a vaule equals null.  the problem is when the second field value is null, the featurelayer render the default value.

[LTHDAFS]

one field UniqueValueRenderer does not work too, when the field value is null.