Emulator: avoid accessing null BluetoothAdapter

bug: 27324609
Emulator has no bluetooth support at this moment, and settings
should not crash in this case.

Change-Id: I1bfc6c907ac02d4798b6c118291e6a9d1bf23c7c

src/com/android/settings/bluetooth/BluetoothSettings.java

@@ -496,8 +496,9 @@ public final class BluetoothSettings extends DeviceListPreferenceFragment implem
 
         @Override
         public void setListening(boolean listening) {
+            BluetoothAdapter defaultAdapter = BluetoothAdapter.getDefaultAdapter();
+            if (defaultAdapter == null) return;
             if (listening) {
-                BluetoothAdapter defaultAdapter = BluetoothAdapter.getDefaultAdapter();
                 mEnabled = defaultAdapter.isEnabled();
                 mConnected =
                         defaultAdapter.getConnectionState() == BluetoothAdapter.STATE_CONNECTED;