Fix b/243746796 Crash when mouse hangs over an empty battery chart

Bug: 243746796
Fix: 243746796
Test: manual
Change-Id: I3e595c96a55f1114b50ed5ed8e49e962cc90cbde
Merged-In: I3e595c96a55f1114b50ed5ed8e49e962cc90cbde

src/com/android/settings/fuelgauge/batteryusage/BatteryChartView.java

@@ -595,6 +595,9 @@ public class BatteryChartView extends AppCompatImageView implements View.OnClick
 
     // Searches the corresponding trapezoid index from x location.
     private int getTrapezoidIndex(float x) {
+        if (mTrapezoidSlots == null) {
+            return BatteryChartViewModel.SELECTED_INDEX_INVALID;
+        }
         for (int index = 0; index < mTrapezoidSlots.length; index++) {
             final TrapezoidSlot slot = mTrapezoidSlots[index];
             if (x >= slot.mLeft - mTrapezoidHOffset