Add null check when restart batteryinfo loader

Even though we unregister contentObserver in onPause and register
in onResume, callback still be called while fragment is detached.

Anyhow add a null check in settings to stop crash

Fixes: 131905853
Test: RunSettingsRoboTests
Change-Id: I8c0c2c04c3b8d942e0c97cf71a7d3e735a24b467

src/com/android/settings/fuelgauge/PowerUsageSummary.java

@@ -378,6 +378,9 @@ public class PowerUsageSummary extends PowerUsageBase implements OnLongClickList
 
     @VisibleForTesting
     void restartBatteryInfoLoader() {
+        if (getContext() == null) {
+            return;
+        }
         getLoaderManager().restartLoader(BATTERY_INFO_LOADER, Bundle.EMPTY,
                 mBatteryInfoLoaderCallbacks);
         if (mPowerFeatureProvider.isEstimateDebugEnabled()) {