尝试在Android Studio中运行程序，但收到错误，我的应用程序已停止工作

问：

每当我尝试运行我的应用程序时，我都会在我的 Android Studio 模拟器 （Nexus_7_API_21） 中收到错误，即我的应用程序已停止工作。我启动了 LogCat，我认为错误是由于空指针引用造成的，但我不知道问题出在哪里。

这是我的 .java 主文件。

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.view.View;
import android.widget.RadioButton;
import android.widget.RelativeLayout;

public class MainActivity extends ActionBarActivity
{
    RelativeLayout bg;
    RadioButton r1;
    RadioButton r2;
    RadioButton r3;
    RadioButton r4;
    Button b;

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

        bg = (RelativeLayout) findViewById(R.id.back);
        r1 = (RadioButton) findViewById(R.id.rButton1);
        r2 = (RadioButton) findViewById(R.id.rButton2);
        r1 = (RadioButton) findViewById(R.id.rButton3);
        r1 = (RadioButton) findViewById(R.id.rButton4);

        r1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
        r2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
        r3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
        r4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                return;
            }
        });
    }

    @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_main, menu);
        return true;
    }

    @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_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

这是我的XML文件（）：activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:id="@+id/back"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity"
    android:background="#210d0a0d">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:layout_alignParentEnd="true">

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="fill_parent"
            android:layout_height="200dp">

            <LinearLayout
                android:orientation="horizontal"
                android:layout_width="fill_parent"
                android:layout_height="35dp" >

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:textAppearance="?android:attr/textAppearanceLarge"
                    android:text="@string/welcomeString"
                    android:id="@+id/textView"
                    android:layout_gravity="center_horizontal"
                    android:textStyle="bold" />
            </LinearLayout>

            <LinearLayout
                android:orientation="horizontal"
                android:layout_width="180dp"
                android:layout_height="150dp" >

                <TableLayout
                    android:layout_width="fill_parent"
                    android:layout_height="fill_parent" >

                    <TableRow
                        android:layout_width="fill_parent"
                        android:layout_height="fill_parent" >

                        <RadioButton
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:text="@string/rButton1"
                            android:id="@+id/rButton1"
                            android:layout_gravity="center_vertical"
                            android:checked="false" />
                    </TableRow>

                    <TableRow
                        android:layout_width="fill_parent"
                        android:layout_height="fill_parent" >

                        <RadioButton
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:text="@string/rButton2"
                            android:id="@+id/rButton2"
                            android:layout_gravity="center_vertical" />
                    </TableRow>

                    <TableRow
                        android:layout_width="fill_parent"
                        android:layout_height="fill_parent" >

                        <RadioButton
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:text="@string/rButton3"
                            android:id="@+id/rButton3"
                            android:layout_gravity="center_vertical" />
                    </TableRow>

                    <TableRow
                        android:layout_width="fill_parent"
                        android:layout_height="fill_parent" >

                        <RadioButton
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:text="@string/rButton4"
                            android:id="@+id/rButton4"
                            android:layout_gravity="center_vertical" />
                    </TableRow>
                </TableLayout>
            </LinearLayout>

        </LinearLayout>

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent">

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="@string/button"
                android:id="@+id/button"
                android:layout_gravity="center_horizontal"
                android:textSize="25dp" />
        </LinearLayout>

    </LinearLayout>

</RelativeLayout>