supplemental accessor for battery reactive power fails if nothing

[claytonpbarrows]

get_max_reactive_power(battery)
ERROR: type Nothing has no field max
Stacktrace:
 [1] getproperty(x::Nothing, f::Symbol)
   @ Base ./Base.jl:33
 [2] get_max_reactive_power(d::GenericBattery)
   @ PowerSystems ~/.julia/packages/PowerSystems/Rjz37/src/models/supplemental_accessors.jl:43