fixed crash in wifi direct when no wifi direct is available

src/de/tudarmstadt/informatik/hostage/sync/wifi_direct/WiFiP2pBroadcastReceiver.java

@@ -226,6 +226,8 @@ public class WiFiP2pBroadcastReceiver extends BroadcastReceiver implements WifiP
      * Disconnects from the connected wifi direct group if the own device is still connected.
      */
     public void disconnect() {
+		if (this.getOwnDevice() == null) return; // no wifi connect
+
         if (isConnected||this.getOwnDevice().status == WifiP2pDevice.CONNECTED){
             manager.removeGroup(channel, new WifiP2pManager.ActionListener() {
                 private WiFiP2pBroadcastListener eventListener;