Check if argument id is null before searching for whether `nearbyUs…

[azanli]

…ershas adeviceIdthat containsid. When (id == null) return nullbecause not doing so causesnearbyUsers.elementAt(i).deviceId.contains(id)to attempt to invoketoString()onid` when it is null resulting in a crash.

The resulting error message:

2019-04-20 21:26:52.578 10574-10642/? E/AndroidRuntime: FATAL EXCEPTION: mqt_native_modules
    Process: com.app, PID: 10574
    java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.String java.lang.CharSequence.toString()' on a null object reference
        at java.lang.String.contains(String.java:2196)
        at com.rctunderdark.NetworkCommunicator.findUser(NetworkCommunicator.java:245)
        at com.rctunderdark.NetworkCommunicator.sendMessage(NetworkCommunicator.java:235)
        at com.rctunderdark.NetworkManager.sendMessage(NetworkManager.java:29)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.facebook.react.bridge.JavaMethodWrapper.invoke(JavaMethodWrapper.java:372)
        at com.facebook.react.bridge.JavaModuleWrapper.invoke(JavaModuleWrapper.java:158)
        at com.facebook.react.bridge.queue.NativeRunnable.run(Native Method)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at com.facebook.react.bridge.queue.MessageQueueThreadHandler.dispatchMessage(MessageQueueThreadHandler.java:29)
        at android.os.Looper.loop(Looper.java:193)
        at com.facebook.react.bridge.queue.MessageQueueThreadImpl$4.run(MessageQueueThreadImpl.java:232)
        at java.lang.Thread.run(Thread.java:764)

Steps to reproduce
This happens when previously connected devices (one being Android, hence the Java fix) disconnect and then without restarting the application, they attempt to reconnect.