Fix a couple of npes caused by user tapping on a chart which has no data

Stack traces for the two NPEs were:

java.lang.NullPointerException: Attempt to invoke virtual method 'int com.github.mikephil.charting.data.ChartData.getDataSetCount()' on a null object reference
            at com.github.mikephil.charting.charts.Chart.getYValsAtIndex(Chart.java:1480)
            at com.github.mikephil.charting.charts.BarLineChartBase.getHighlightByTouchPoint(BarLineChartBase.java:1523)
            at com.github.mikephil.charting.listener.BarLineChartTouchListener.onSingleTapConfirmed(BarLineChartTouchListener.java:380)
            at android.view.GestureDetector$GestureHandler.handleMessage(GestureDetector.java:273)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5070)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:836)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:631)


and

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.ArrayList com.github.mikephil.charting.data.ChartData.getDataSets()' on a null object reference
            at com.github.mikephil.charting.data.DrawingContext.deleteLastDrawingEntry(DrawingContext.java:151)
            at com.github.mikephil.charting.listener.BarLineChartTouchListener.onTouch(BarLineChartTouchListener.java:111)
            at com.github.mikephil.charting.charts.Chart.onTouchEvent(Chart.java:746)
            at android.view.View.dispatchTouchEvent(View.java:8073)
            at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2259)
            at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2001)
            at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2259)
            at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2001)
            at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2259)
            at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2001)
            at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2259)
            at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2001)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2198)
            at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1649)
            at android.app.Activity.dispatchTouchEvent(Activity.java:2717)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2159)
            at android.view.View.dispatchPointerEvent(View.java:8263)
            at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4013)
            at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3892)
            at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3454)
            at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3507)
            at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3473)
            at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3583)
            at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3481)
            at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3640)
            at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3454)
            at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3507)
            at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3473)
            at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3481)
            at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3454)
            at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5682)
            at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5656)
            at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5627)
            at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:5761)
            at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
            at android.os.MessageQueue.nativePollOnce(Native Method)
            at android.os.MessageQueue.next(MessageQueue.java:143)
            at android.os.Looper.loop(Looper.java:122)
            at android.app.ActivityThread.main(ActivityThread.java:5070)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:836)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:631)