Prevent crash when updating datasets and calling animation functions

Sometimes, when changing datasets and then animating, an erroneous highlight is generated for the removed dataset. Null check prevents crash (interestingly the iOS lib had the same issue)

Author: StainlessStlRat

MPChartLib/src/main/java/com/github/mikephil/charting/charts/Chart.java

@@ -761,8 +761,10 @@ protected void drawMarkers(Canvas canvas) {
 
 		for (Highlight highlight : mIndicesToHighlight) {
 
+			// When changing data sets and calling animation functions, sometimes an erroneous highlight is generated
+			// on the dataset that is removed. Null check to prevent crash
 			IDataSet set = mData.getDataSetByIndex(highlight.getDataSetIndex());
-			if (!set.isVisible()) {
+			if (set == null || !set.isVisible()) {
 				continue;
 			}