Bottom Navigation View with activities not working in Kotlin - android-studio

Question

I am new to Kotlin, so I am having trouble on how to solve this.
I have created a bottom navigation view that I am using to switch between activities. When I call the BottomNavigationView.OnNavigationItemSelectedListener my app crashes and I dont know why. Can anyone please help?
I have searched, but I only find answers on fragments.
My MainActivity code:
class MainActivity : AppCompatActivity() {
private val navigasjonen = BottomNavigationView.OnNavigationItemSelectedListener { item ->
when (item.itemId) {
R.id.home -> {
return#OnNavigationItemSelectedListener false
}
R.id.leggTilNavigasjon -> {
val intent = Intent(this#MainActivity, AddRec::class.java)
startActivity(intent)
return#OnNavigationItemSelectedListener true
}
R.id.favoritter -> {
val intent = Intent(this#MainActivity, MyRecipes::class.java)
startActivity(intent)
return#OnNavigationItemSelectedListener true
}
R.id.utforsk -> {
return#OnNavigationItemSelectedListener true
}
}
false
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val bottomNavigation = findViewById<BottomNavigationView>(R.id.bottom_navigation)
//bottomNavigation.setSelectedItemId(R.id.home)
bottomNavigation.setOnNavigationItemSelectedListener(navigasjonen)
}
}
My xml code:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:background="#color/colorLys"
android:id="#+id/home">
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="#+id/navigationView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
android:background="?android:attr/windowBackground"
app:menu="#menu/navigasjon" />
</androidx.constraintlayout.widget.ConstraintLayout>
My error:
2020-01-27 14:20:06.197 20309-20309/com.example.foodleaks E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.foodleaks, PID: 20309
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.foodleaks/com.example.foodleaks.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
at com.example.foodleaks.MainActivity.onCreate(MainActivity.kt:39)
at android.app.Activity.performCreate(Activity.java:7136)
at android.app.Activity.performCreate(Activity.java:7127)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
at android.os.Handler.dispatchMessage(Handler.java:106) 
at android.os.Looper.loop(Looper.java:193) 
at android.app.ActivityThread.main(ActivityThread.java:6669) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

Answer 1

In your findViewById you are passing R.id.bottom_navigation:
val bottomNavigation = findViewById<BottomNavigationView>(R.id.bottom_navigation)
and in the layout it has the id:
android:id="#+id/navigationView"
You can either change the id on the layout:
android:id="#+id/bottom_navigation"
Or change it in the findViewById:
val bottomNavigation = findViewById<BottomNavigationView>(R.id.navigationView)
Be consistent so that it can find the BottomNavigationView element and it won't crash for a null object reference.

Answer 2

The error is present in the stacktrace. I recommend learning to read Java/Android stacktraces :)
The culprit in stacktrace:
Attempt to invoke virtual method 'void com.google.android.material.bottomnavigation.BottomNavigationView.setOnNavigationItemSelectedListener(com.google.android.material.bottomnavigation.BottomNavigationView$OnNavigationItemSelectedListener)' on a null object reference
Your BottomNavigationView is null. Why?
Look at your declaration in layout:
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="#+id/navigationView
And that's how you find the view:
findViewById<BottomNavigationView>(R.id.bottom_navigation)
You are using wrong view id.