Add null check for vrManager

If VrManagerService is not started, vrManager will be null. Need
add check for it.

Test: robotests
Bug: 112064298
Change-Id: I899337bb5a996efffe82970fa690f2c5d59c1bb5

tests/robotests/src/com/android/settings/display/BrightnessLevelPreferenceControllerTest.java

@@ -82,6 +82,12 @@ public class BrightnessLevelPreferenceControllerTest {
         assertThat(mController.isAvailable()).isTrue();
     }
 
+    @Test
+    public void isInVrMode_noVrManager_shouldAlwaysReturnFalse() {
+        doReturn(null).when(mController).safeGetVrManager();
+        assertThat(mController.isInVrMode()).isFalse();
+    }
+
     @Test
     public void onStart_shouldRegisterObserver() {
         BrightnessLevelPreferenceController controller =