Fix NPE in rotation lock content observer.

Bug: 6647846
Change-Id: Idd17edb78f451807eabdd988017b68ce02b9c75f
diff --git a/src/com/android/settings/AccessibilitySettings.java b/src/com/android/settings/AccessibilitySettings.java
index 149e336..6167f78 100644
--- a/src/com/android/settings/AccessibilitySettings.java
+++ b/src/com/android/settings/AccessibilitySettings.java
@@ -449,8 +449,11 @@
     }
 
     private void updateLockScreenRotationCheckbox() {
-        mToggleLockScreenRotationPreference.setChecked(
-                !RotationPolicy.isRotationLocked(getActivity()));
+        Context context = getActivity();
+        if (context != null) {
+            mToggleLockScreenRotationPreference.setChecked(
+                    !RotationPolicy.isRotationLocked(context));
+        }
     }
 
     private void offerInstallAccessibilitySerivceOnce() {