Fixed NullPointerException when using Slider on Android 2.3.3

[iamovrhere]

On a 2.3.3 device an application throws a NullPointerException. The problem does not present itself on a 4.3 test device. Wrapping the ball.invalidate() call in a null check seems to resolve it.

Original trace below:
E/AndroidRuntime( 3827): android.view.InflateException: Binary XML file line #9: Error inflating class com.gc.materialdesign.views.Slider E/AndroidRuntime( 3827): at android.view.LayoutInflater.createView(LayoutInflater.java:518) E/AndroidRuntime( 3827): at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:570) E/AndroidRuntime( 3827): at android.view.LayoutInflater.rInflate(LayoutInflater.java:623) E/AndroidRuntime( 3827): at android.view.LayoutInflater.inflate(LayoutInflater.java:408) E/AndroidRuntime( 3827): at android.view.LayoutInflater.inflate(LayoutInflater.java:320) E/AndroidRuntime( 3827): at android.view.ViewStub.inflate(ViewStub.java:232) E/AndroidRuntime( 3827): at android.view.ViewStub.setVisibility(ViewStub.java:213) E/AndroidRuntime( 3827): .... E/AndroidRuntime( 3827): at android.os.Handler.dispatchMessage(Handler.java:99) E/AndroidRuntime( 3827): at android.os.Looper.loop(Looper.java:130) E/AndroidRuntime( 3827): at android.app.ActivityThread.main(ActivityThread.java:3687) E/AndroidRuntime( 3827): at java.lang.reflect.Method.invokeNative(Native Method) E/AndroidRuntime( 3827): at java.lang.reflect.Method.invoke(Method.java:507) E/AndroidRuntime( 3827): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) E/AndroidRuntime( 3827): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625) E/AndroidRuntime( 3827): at dalvik.system.NativeStart.main(Native Method) E/AndroidRuntime( 3827): Caused by: java.lang.reflect.InvocationTargetException E/AndroidRuntime( 3827): at java.lang.reflect.Constructor.constructNative(Native Method) E/AndroidRuntime( 3827): at java.lang.reflect.Constructor.newInstance(Constructor.java:415) E/AndroidRuntime( 3827): at android.view.LayoutInflater.createView(LayoutInflater.java:505) E/AndroidRuntime( 3827): ... 30 more E/AndroidRuntime( 3827): Caused by: java.lang.NullPointerException E/AndroidRuntime( 3827): at com.gc.materialdesign.views.Slider.invalidate(Slider.java:97) E/AndroidRuntime( 3827): at android.view.View.setBackgroundDrawable(View.java:7626) E/AndroidRuntime( 3827): at android.view.View.<init>(View.java:2160) E/AndroidRuntime( 3827): at android.view.View.<init>(View.java:1899) E/AndroidRuntime( 3827): at android.view.ViewGroup.<init>(ViewGroup.java:286) E/AndroidRuntime( 3827): at android.widget.RelativeLayout.<init>(RelativeLayout.java:173) E/AndroidRuntime( 3827): at com.gc.materialdesign.views.CustomView.<init>(CustomView.java:22) E/AndroidRuntime( 3827): at com.gc.materialdesign.views.Slider.<init>(Slider.java:41) E/AndroidRuntime( 3827): ... 33 more