Fix b/262378515: NullPointerException in TabPreference.onDetached()

Bug: 262378515
Fix: 262378515
Test: presubmit
Change-Id: I9c0629d570538a664c361eae74e8f7818f4210df

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

@@ -87,7 +87,7 @@ public class TabPreference extends Preference {
     @Override
     public void onDetached() {
         super.onDetached();
-        if (mOnPageChangeCallback != null) {
+        if (mViewPager != null && mOnPageChangeCallback != null) {
             mViewPager.unregisterOnPageChangeCallback(mOnPageChangeCallback);
         }
     }