Add null check to batterymeterview in case the percent view becomes null

Seems like this can only happen if we have a successful battery reading and
then immediately the conditions change causing us to remove the precent
view.

Test: atest SystemUITests
Fixes: 180019511
Change-Id: I4bf8508a9cef18ade62863b07086254e39897059

packages/SystemUI/src/com/android/systemui/BatteryMeterView.java

@@ -349,6 +349,9 @@ public class BatteryMeterView extends LinearLayout implements
     }
 
     private void setPercentTextAtCurrentLevel() {
+        if (mBatteryPercentView == null) {
+            return;
+        }
         mBatteryPercentView.setText(
                 NumberFormat.getPercentInstance().format(mLevel / 100f));
         setContentDescription(