Crash when created adapter for wheelview

[DoaiNguyen]

when i created a class was extended NumericWheelAdapter, ArrayWheelAdapter and used it.
Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
.widget.adapters.AbstractWheelTextAdapter.void (android.content.Context)(Unknown Source)
void (android.content.Context,int)
void (android.content.Context,int,int)
.widget.adapters.AbstractWheelTextAdapter.void (android.content.Context)(Unknown Source)
void (android.content.Context,int)
void (android.content.Context,int,int)
.widget.adapters.AbstractWheelTextAdapter.void (android.content.Context)(Unknown Source)
void (android.content.Context,int)
void (android.content.Context,int,int)
.widget.adapters.NumericWheelAdapter.void (android.content.Context,int,int)(Unknown Source)
void (android.content.Context,int,int,java.lang.String)
.widget.adapters.NumericWheelAdapter.void (android.content.Context,int,int)(Unknown Source)
void (android.content.Context,int,int,java.lang.String)