why am i getting nullpointexception on a virtual method onclick listener - onclicklistener

Question

I'm trying to implement a popup window for app, but I guess my implementation is not correct. I'm getting nullpointexception virtual method null object. I guess I'm calling the onclickListener on a null object. what is the correct way to implement the popup window. The window responds to a click on a menu item "about" Any help appreciated.
exception error
Process: com.example.android.droidscanner, PID: 14958
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.android.droidscanner.MainActivity.showAboutWindow(MainActivity.java:152)
at com.example.android.droidscanner.MainActivity.onOptionsItemSelected(MainActivity.java:131)
MainActivity
package com.example.android.droidscanner;
import android.content.Intent;
import android.graphics.Color;
import android.net.wifi.WifiInfo;
import android.net.wifi.WifiManager;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.PopupWindow;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity {
Button btnReadNet;
TextView textResult;
ListView listViewNode;
ArrayList<String> networkList;
ArrayAdapter<String> adapterNet;
public String ip;
Toolbar myToolbar;
private LayoutInflater popUpInflater;
private PopupWindow popUpWindow;
private Button btnDismiss;
private LinearLayout linearMain;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//setting the ActionBar for the activity
myToolbar = (Toolbar) findViewById(R.id.my_toolbar);
setSupportActionBar(myToolbar);
btnReadNet = (Button) findViewById(R.id.read_network);
textResult = (TextView) findViewById(R.id.result);
//referencing listview from activity_main.xml
listViewNode = (ListView) findViewById(R.id.nodelist);
networkList = new ArrayList<>();
//referencing LinearLayout in activity_main.xml for popup window
linearMain = (LinearLayout) findViewById(R.id.linearLayout_main);
//inflating the adapter
adapterNet = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, networkList);
listViewNode.setAdapter(adapterNet);
WifiManager wifiMan = (WifiManager) getSystemService(WIFI_SERVICE);
WifiInfo wifiInfo = wifiMan.getConnectionInfo();
int ipAddress = wifiInfo.getIpAddress();
ip = String.format("%d.%d.%d.%d", (ipAddress & 0xff), (ipAddress >> 8 & 0xff), (ipAddress >> 16 & 0xff), (ipAddress >> 24 & 0xff));
textResult.setText(printOutput(wifiInfo));
//starting network scanner activity on button press
btnReadNet.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent scanNetIntent = new Intent(MainActivity.this, IpScanActivity.class);
scanNetIntent.putExtra("ip", ip);
startActivity(scanNetIntent);
}
});
}
public String printOutput(WifiInfo wifiInfo) {
return "Local IP: " + ip + "\n" +
"SSID: " + wifiInfo.getSSID() + "\n" +
"BSSID: " + wifiInfo.getBSSID() + "\n" +
"MAC: " + wifiInfo.getMacAddress() + "\n" +
"Signal: " + wifiInfo.getRssi() + "db" + "\n" +
"Speed: " + wifiInfo.getLinkSpeed() + "Mbps" + "\n" +
//
"Connection: " + wifiInfo.getSupplicantState();
}
/*
* Infating the menu for toolbar
*/
#Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu, menu);
return true;
}
/*
* actions for menu options in toolbar
*/
#Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_ping) {
Intent pingIntent = new Intent(MainActivity.this, PingActivity.class);
startActivity(pingIntent);
return true;
}
if (id == R.id.action_netstat) {
Intent netstatIntent = new Intent(MainActivity.this, NetstatActivity.class);
startActivity(netstatIntent);
return true;
}
if (id == R.id.action_refresh) {
//reload this activity (refresh)
Intent thisActivity = new Intent(this, MainActivity.class);
startActivity(thisActivity);
return true;
}
if (id == R.id.action_about) {
showAboutWindow();
return true;
}
return super.onOptionsItemSelected(item);
}
/*
* method to display about popup window with app informmation
* eg: developer name, version, website
*/
public void showAboutWindow() {
//layout inflater for about popup window
popUpInflater = (LayoutInflater) getApplicationContext().getSystemService(LAYOUT_INFLATER_SERVICE);
ViewGroup container = (ViewGroup) popUpInflater.inflate(R.layout.popup_win, null);
popUpWindow = new PopupWindow(container, 500, 500, true);
popUpWindow.showAtLocation(linearMain, Gravity.NO_GRAVITY, 500, 500);
//button reference for popup window dismiss button
btnDismiss = (Button) findViewById(R.id.dismiss);
btnDismiss.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
popUpWindow.dismiss();
}
});
}
}
popup window xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical">
<TextView
android:id="#+id/app_info"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="DroidScanner by zentech"
android:textSize="18dp"/>
<TextView
android:id="#+id/url_info"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="http://pctechtips.org"
android:textSize="18dp"/>
<Button
android:id="#+id/dismiss"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:textSize="18dp"
android:text="ok" />
</LinearLayout>
</LinearLayout>