fix NPE when use solid color with barchart

[oatrice]

PR Checklist:

• I have tested this extensively and it does not break any existing behavior.
• I have added/updated examples and tests for any new behavior.
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]
  

PR Description

fix NPE when use solid color with barchart

2020-02-07 18:01:27.307 10747-10747/com.xxmassdeveloper.mpchartexample E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.xxmassdeveloper.mpchartexample, PID: 10747
    java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at com.github.mikephil.charting.renderer.BarChartRenderer.drawDataSet(BarChartRenderer.java:147)
        at com.github.mikephil.charting.renderer.BarChartRenderer.drawData(BarChartRenderer.java:81)
        at com.github.mikephil.charting.charts.BarLineChartBase.onDraw(BarLineChartBase.java:242)
        at android.view.View.draw(View.java:20617)
        at android.view.View.updateDisplayListIfDirty(View.java:19449)
        at android.view.View.draw(View.java:20330)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4508)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4264)
        at android.view.View.updateDisplayListIfDirty(View.java:19440)
        at android.view.View.draw(View.java:20330)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4508)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4264)
        at android.view.View.updateDisplayListIfDirty(View.java:19440)
        at android.view.View.draw(View.java:20330)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4508)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4264)
        at android.view.View.updateDisplayListIfDirty(View.java:19440)
        at android.view.View.draw(View.java:20330)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4508)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4264)
        at android.view.View.updateDisplayListIfDirty(View.java:19440)
        at android.view.View.draw(View.java:20330)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4508)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4264)
        at android.view.View.updateDisplayListIfDirty(View.java:19440)
        at android.view.View.draw(View.java:20330)
        at android.view.ViewGroup.drawChild(ViewGroup.java:4508)
        at android.view.ViewGroup.dispatchDraw(ViewGroup.java:4264)
        at android.view.View.draw(View.java:20625)
        at com.android.internal.policy.DecorView.draw(DecorView.java:916)
        at android.view.View.updateDisplayListIfDirty(View.java:19449)
        at android.view.ThreadedRenderer.updateViewTreeDisplayList(ThreadedRenderer.java:690)
        at android.view.ThreadedRenderer.updateRootDisplayList(ThreadedRenderer.java:696)
        at android.view.ThreadedRenderer.draw(ThreadedRenderer.java:805)
        at android.view.ViewRootImpl.draw(ViewRootImpl.java:3975)
        at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:3750)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:3046)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1815)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:8228)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:1048)
        at android.view.Choreographer.doCallbacks(Choreographer.java:860)
        at android.view.Choreographer.doFrame(Choreographer.java:792)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:1034)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:226)
        at android.app.ActivityThread.main(ActivityThread.java:7212)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:576)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:956)

[danielgindi]

Please clean it up a bit

[oatrice]

thank, edited