following problems occurred during testing of app on Mi device(Redmi Note 3)

1. Host server ip fetch gives incorrect ip.
2. connection with server from client Activity destroys server.

Here is the stack trace:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.kmlkant3497.bru_chat, PID: 27150
java.lang.RuntimeException: Unable to stop activity {com.example.kmlkant3497.bru_chat/com.example.kmlkant3497.bru_chat.HostTabActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void java.net.ServerSocket.close()' on a null object reference
at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3780)
at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3834)
at android.app.ActivityThread.access$1500(ActivityThread.java:164)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1431)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:150)
at android.app.ActivityThread.main(ActivityThread.java:5408)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void java.net.ServerSocket.close()' on a null object reference
at com.example.kmlkant3497.bru_chat.HostTabActivity.onStop(HostTabActivity.java:138)
at android.app.Instrumentation.callActivityOnStop(Instrumentation.java:1277)
at android.app.Activity.performStop(Activity.java:6237)
at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:3775)
at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:3834) 
at android.app.ActivityThread.access$1500(ActivityThread.java:164) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1431) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:150) 
at android.app.ActivityThread.main(ActivityThread.java:5408) 
at java.lang.reflect.Method.invoke(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:372) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:964) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759) 

Edit:
the error occurs around this line of code, (in the serverSocket.close())

@Override
    protected void onStop() {
        super.onStop();
        try {
            // MAKE SURE YOU CLOSE THE SOCKET UPON EXITING
            serverSocket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }