LuckyWheel crashes when it has no WheelItems to draw

[mikeblas]

Steps to reproduce:

1. In Android Studio, create a new "Empty Activity" project
2. Per the LuckyWheel README.md, add the dependencies to the Gradle scripts
3. Open the activity_main.xml layout resource and add this XML block:

<com.bluehomestudio.luckywheel.LuckyWheel
        android:id="@+id/lwv"
        android:layout_width="250dp"
        android:layout_height="250dp"
        android:layout_centerInParent="true" />

4. With no other changes, build and run the application.

BUG: The application starts, then immediately crashes with this call stack:

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.wheeltest1, PID: 20695
    java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at com.bluehomestudio.luckywheel.WheelView.onDraw(WheelView.java:240)
        at android.view.View.draw(View.java:19123)
        at android.view.View.updateDisplayListIfDirty(View.java:18073)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at android.view.View.updateDisplayListIfDirty(View.java:18064)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at android.view.View.updateDisplayListIfDirty(View.java:18064)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at androidx.constraintlayout.widget.ConstraintLayout.dispatchDraw(ConstraintLayout.java:1975)
        at android.view.View.updateDisplayListIfDirty(View.java:18064)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at android.view.View.updateDisplayListIfDirty(View.java:18064)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at android.view.View.updateDisplayListIfDirty(View.java:18064)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at android.view.View.updateDisplayListIfDirty(View.java:18064)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at android.view.View.updateDisplayListIfDirty(View.java:18064)
        at android.view.View.draw(View.java:18851)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4214)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4000)
        at android.view.View.draw(View.java:19126)
        at com.android.internal.policy.DecorView.draw(DecorView.java:785)
        at android.view.View.updateDisplayListIfDirty(View.java:18073)
        at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:643)
        at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:649)
        at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:757)
        at android.view.ViewRootImpl.draw(ViewRootImpl.java:2980)
        at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:2794)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2347)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1386)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6733)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
        at android.view.Choreographer.doCallbacks(Choreographer.java:723)
        at android.view.Choreographer.doFrame(Choreographer.java:658)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
        at android.os.Handler.handleCallback(Handler.java:789)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6541)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

The problem is that the LuckyWheel doesn't check to see if the list of WheelItems is empty. While that's not a great use case, sometimes in development not all steps are completed. The control would offer developers a better experience if it was able to draw itself without crashing when it contained no WheelItems. Perhaps just a circle would be fine -- and maybe log a warning that there were no items in the control to be drawn.

[mmoamenn]

PR in review