无法实例化：NullPointerException：尝试在空obj

Question

我正在尝试构建一个可从MySQL数据库（由树莓派托管）访问表的android应用程序。并不断收到此错误...我知道我没有初始化活动，也许没有正确设置我的OnCreate ...仍然不确定我遇到的具体错误以及如何解决此错误。堆栈跟踪以及我编写的一些代码：

    I/InstantRun: starting instant run server: is main process
    D/AndroidRuntime: Shutting down VM
    E/AndroidRuntime: FATAL EXCEPTION: main
        Process: com.notes.app_transcribe, PID: 8764
        java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.notes.app_transcribe/com.notes.app_transcribe.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference

  I/InstantRun: starting instant run server: is main process
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.notes.app_transcribe, PID: 8764
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.notes.app_transcribe/com.notes.app_transcribe.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2819)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2988)
        at android.app.ActivityThread.-wrap14(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1631)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6682)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.support.v7.app.AppCompatDelegateImpl.<init>(AppCompatDelegateImpl.java:249)
        at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:182)
        at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:520)
        at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:191)
        at com.notes.app_transcribe.MainActivity.<init>(MainActivity.java:23)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1086)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2809)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2988) 
        at android.app.ActivityThread.-wrap14(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1631) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6682) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)

package com.notes.app_transcribe;


import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;

public class MainActivity extends AppCompatActivity {

    private static final String url = "jdbc:mysql://10.57.158.209/NoteDB";
    private static final String user = "newuser";
    private static final String pass = "password";
    Button btnFetch = findViewById(R.id.btnFetch),btnClear = findViewById(R.id.btnClear);
    TextView txtData;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        txtData = this.findViewById(R.id.txtData);
        btnFetch.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
// TODO Auto-generated method stub
                ConnectMySql connectMySql = new ConnectMySql();
                connectMySql.execute("");
            }
        });
        btnClear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                txtData.setText("");
            }
        });

    }

    private class ConnectMySql extends AsyncTask<String, Void, String> {
        String res = "";

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            Toast.makeText(MainActivity.this, "Please wait...", Toast.LENGTH_SHORT)
                    .show();

        }

        @Override
        protected String doInBackground(String... params) {
            try {
                Class.forName("com.mysql.jdbc.Driver");
                Connection con = DriverManager.getConnection(url, user, pass);
                System.out.println("Databaseection success");

                String result = "Database Connection Successful\n";
                Statement st = con.createStatement();
                ResultSet rs = st.executeQuery("Notes detected from NoteDB");
                ResultSetMetaData rsmd = rs.getMetaData();

                while (rs.next()) {
                    result += rs.getString(1) + "\n";
                }
                res = result;
            } catch (Exception e) {
                e.printStackTrace();
                res = e.toString();
            }
            return res;
        }

        @Override
        protected void onPostExecute(String result) {
            txtData.setText(result);
        }
    }

这是清单，尽管我相当确定这里没有天生的错误：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="com.notes.app_transcribe">

    <uses-permission android:name="android.permission.INTERNET">

    </uses-permission>
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE">

    </uses-permission>
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE">

    </uses-permission>


    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        tools:ignore="GoogleAppIndexingWarning">

        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>


</manifest>

出于某种原因，我已经研究了此问题，并尝试了多种方法来解决该问题，但无济于事。仍然无法找到包含答案的“重复”问题，该问题解释了我如何能够纠正错误。