title: Android app keeps crashing in emulator
body: <p>I am making my first android app.
  Android Studio shows no error in the code.
  However, the app keeps crashing as soon as I try to launch it in the Emulator.
  I am using the inbuilt Nexus 5X API 23 for the emulator.
  Please help me?
  Thank you!</p>
  
  <p>Here is my Manifest file:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.example.admin.coursebrowser"&gt;
  
  &lt;application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:supportsRtl="true"
      android:theme="@style/AppTheme"&gt;
      &lt;activity
          android:name=".MainActivity"
          android:label="@string/title_activity_main"
          android:theme="@style/AppTheme.NoActionBar"&gt;
          &lt;intent-filter&gt;
              &lt;action android:name="android.intent.action.MAIN" /&gt;
  
              &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
          &lt;/intent-filter&gt;
      &lt;/activity&gt;
  &lt;/application&gt;
  </code></pre>
  
  <p></p>
  
  <p>Here is my MainActivity.java file:</p>
  
  <pre><code>package com.example.admin.coursebrowser;
  
  import android.os.Bundle;
  import android.support.design.widget.FloatingActionButton;
  import android.support.design.widget.Snackbar;
  import android.support.v7.app.AppCompatActivity;
  import android.support.v7.widget.Toolbar;
  import android.view.View;
  import android.widget.RelativeLayout;
  import android.widget.Button;
  import android.graphics.Color;
  
  public class MainActivity extends AppCompatActivity {
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      RelativeLayout myLayout= new RelativeLayout(this);
      myLayout.setBackgroundColor(Color.CYAN);
      Button redButton= new Button(this);
      redButton.setText("Click Me");
      redButton.setBackgroundColor(Color.RED);
      myLayout.addView(redButton);
      setContentView(myLayout);
  
      Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
      setSupportActionBar(toolbar);
  
      FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
      fab.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
              Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                      .setAction("Action", null).show();
          }
      });
  }
  
  }
  </code></pre>
  
  <p>Here is the activity_main.xml file:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.design.widget.CoordinatorLayout      xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:fitsSystemWindows="true"
  tools:context="com.example.admin.coursebrowser.MainActivity"&gt;
  
  &lt;android.support.design.widget.AppBarLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:theme="@style/AppTheme.AppBarOverlay"&gt;
  
      &lt;android.support.v7.widget.Toolbar
          android:id="@+id/toolbar"
          android:layout_width="match_parent"
          android:layout_height="?attr/actionBarSize"
          android:background="?attr/colorPrimary"
          app:popupTheme="@style/AppTheme.PopupOverlay" /&gt;
  
  &lt;/android.support.design.widget.AppBarLayout&gt;
  
  &lt;include layout="@layout/content_main" /&gt;
  
  &lt;android.support.design.widget.FloatingActionButton
      android:id="@+id/fab"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_gravity="bottom|end"
      android:layout_margin="@dimen/fab_margin"
      android:src="@android:drawable/ic_dialog_email" /&gt;
  
  &lt;/android.support.design.widget.CoordinatorLayout&gt;
  </code></pre>
  
  <p>Here is the logcat:</p>
  
  <pre><code>08-10 15:09:23.291 3442-3442/com.example.admin.coursebrowser E/AndroidRuntime: FATAL EXCEPTION: main
     Process: com.example.admin.coursebrowser, PID: 3442
     java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.admin.coursebrowser/com.example.admin.coursebrowser.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
         at android.app.ActivityThread.-wrap11(ActivityThread.java)
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
         at android.os.Handler.dispatchMessage(Handler.java:102)
         at android.os.Looper.loop(Looper.java:148)
         at android.app.ActivityThread.main(ActivityThread.java:5417)
         at java.lang.reflect.Method.invoke(Native Method)
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
      Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
         at com.example.admin.coursebrowser.MainActivity.onCreate(MainActivity.java:30)
         at android.app.Activity.performCreate(Activity.java:6237)
         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
         at android.app.ActivityThread.-wrap11(ActivityThread.java) 
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
         at android.os.Handler.dispatchMessage(Handler.java:102) 
         at android.os.Looper.loop(Looper.java:148) 
         at android.app.ActivityThread.main(ActivityThread.java:5417) 
         at java.lang.reflect.Method.invoke(Native Method) 
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
  </code></pre>