Bug fix bar chart crash

Author: hannesa2

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

@@ -283,6 +283,9 @@ public void setRoundedBarRadius(float mRoundedBarRadius) {
     public String getAccessibilityDescription() {
 
         BarData barData = getBarData();
+        if (barData == null) {
+            return "";
+        }
 
         int entryCount = barData.getEntryCount();