Add a test for the NPE case

Fix: 305784475

Test: unit test
Change-Id: I28b19da80e83bbbddf8a71399fbf706fcbb81a5c

tests/robotests/src/com/android/settings/bluetooth/BluetoothDetailsDataSyncControllerTest.java

@@ -91,6 +91,14 @@ public class BluetoothDetailsDataSyncControllerTest extends BluetoothDetailsCont
         assertThat(mController.isAvailable()).isTrue();
     }
 
+    @Test
+    public void refresh_noAssociations_checkPreferenceInvisible() {
+        mController.mAssociationId = DUMMY_ASSOCIATION_ID;
+        mController.refresh();
+
+        assertThat(mPermSyncPreference.isVisible()).isFalse();
+    }
+
     @Test
     public void refresh_permSyncNull_checkPreferenceInvisible() {
         mPermissionSyncRequest = null;