title: android.content.ActivityNotFoundException: Unable to find explicit activity class Android Studio Kotlin Error
body: <p>so i am new at android studio.
  so i have 4 activities and on mainactivity i have used startActivity method to open Registration activity.
  now when ever i try to run the app the mainactivity is shown but when i click the button "Registration" (i have used setonclicklistener for startactivity) when i click on the button my app crashes and following message is shown </p>
  
  <pre><code> 2020-03-30 02:04:35.435 20647-20647/com.example.airlineticketreservation E/AndroidRuntime: FATAL 
   EXCEPTION: main
  Process: com.example.airlineticketreservation, PID: 20647
  android.content.ActivityNotFoundException: Unable to find explicit activity class 
       {com.example.airlineticketreservation/int}; have you declared this activity in your 
       AndroidManifest.xml?
      at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:1933)
      at android.app.Instrumentation.execStartActivity(Instrumentation.java:1616)
      at android.app.Activity.startActivityForResult(Activity.java:4487)
      at androidx.fragment.app.FragmentActivity.startActivityForResult(FragmentActivity.java:676)
      at android.app.Activity.startActivityForResult(Activity.java:4445)
      at androidx.fragment.app.FragmentActivity.startActivityForResult(FragmentActivity.java:663)
      at android.app.Activity.startActivity(Activity.java:4806)
      at android.app.Activity.startActivity(Activity.java:4774)
      at com.example.airlineticketreservation.MainActivity$onCreate$1.onClick(MainActivity.kt:27)
      at android.view.View.performClick(View.java:6294)
      at android.view.View$PerformClick.run(View.java:24770)
      at android.os.Handler.handleCallback(Handler.java:790)
      at android.os.Handler.dispatchMessage(Handler.java:99)
      at android.os.Looper.loop(Looper.java:164)
      at android.app.ActivityThread.main(ActivityThread.java:6494)
      at java.lang.reflect.Method.invoke(Native Method)
      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
  </code></pre>
  
  <p>and my androidmanifest file is this</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.airlineticketreservation"&gt;
  
  &lt;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"&gt;
  
      &lt;activity android:name=".MainActivity"&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;activity android:name=".Registration"
          android:label="@string/app_name"
          /&gt;
  
  
  
  
  
      &lt;meta-data
          android:name="preloaded_fonts"
          android:resource="@array/preloaded_fonts" /&gt;
  &lt;/application&gt;
  
  &lt;/manifest&gt;
  </code></pre>
  
  <p>and my mainactivity.kt  file is this</p>
  
  <pre><code>package com.example.airlineticketreservation
  
     import android.content.Intent
     import androidx.appcompat.app.AppCompatActivity
     import android.os.Bundle
     import android.widget.Button
     import android.widget.Spinner
  
  
    class MainActivity : AppCompatActivity() {
  
  override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
  
        /* var book_button: Button = findViewById(R.id.book_ticket)
         var name_view:EditText = findViewById(R.id.name_field)
         var cnic_view:EditText = findViewById(R.id.cnic_field)
          var cancellationBtn_mainActivity: Button = findViewById(R.id.cancel_ticket_btn)
         var spinner:Spinner = findViewById(R.id.spinner)
  
         */
         var registrationBtn_mainActivity : Button = findViewById(R.id.register_btn)
  
  registrationBtn_mainActivity.setOnClickListener {
             var intentx = Intent(this,R.layout.registration::class.java)
             startActivity(intentx)
  
         }
  </code></pre>
  
  <p>what should i do?</p>