Fix NPE in Display Settings.

Handle occasional race in delivering change notifications.

Bug: 6012882
Change-Id: Ic9eff72920ff65a2c8ab85f4af59ba438cd8f99f

src/com/android/settings/DisplaySettings.java

@@ -236,6 +236,7 @@ public class DisplaySettings extends SettingsPreferenceFragment implements
     }
 
     private void updateAccelerometerRotationCheckbox() {
+        if (getActivity() == null) return;
         mAccelerometer.setChecked(Settings.System.getInt(
                 getContentResolver(),
                 Settings.System.ACCELEROMETER_ROTATION, 0) != 0);