This is not allowed in android. You are only allowed to find a view that you can see in the UI. Try using the binding for the SearchView as your content.
Example:

binding = SearchViewBinding.inflate(getLayoutInflater());
setContentView(binding.getRoot());

I want to include a SearchView I have found onto my Android app, but I am facing an issue when setAdapter binds the adapter to the view:

OnCreate from MainActivity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    binding = ActivityMainBinding.inflate(getLayoutInflater());
    setContentView(binding.getRoot());




    // Locate the ListView in listview_main.xml
    list = findViewById(R.id.listview);

    for (int i = 0; i < animalNameList.length; i++) {
        AnimalNames animalNames = new AnimalNames(animalNameList[i]);
        // Binds all strings into an array
        arraylist.add(animalNames);
    }

    // Pass results to ListViewAdapter Class
    adapter = new ListViewAdapter(this, arraylist);

    // Binds the Adapter to the ListView
    list.setAdapter(adapter);

    // Locate the EditText in listview_main.xml
    editsearch = findViewById(R.id.search);
    editsearch.setOnQueryTextListener(this);



    setSupportActionBar(binding.appBarMain.toolbar);

    DrawerLayout drawer = binding.drawerLayout;
    NavigationView navigationView = binding.navView;
    // Passing each menu ID as a set of Ids because each
    // menu should be considered as top level destinations.
    mAppBarConfiguration = new AppBarConfiguration.Builder(R.id.nav_main,R.id.nav_add_alumno,
            R.id.nav_alumnado, R.id.nav_ejercicios, R.id.nav_insignias, R.id.nav_settings)
            .setOpenableLayout(drawer)
            .build();
    NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment_content_main);
    NavController navController = navHostFragment.getNavController();
    NavigationUI.setupActionBarWithNavController(this, navController, mAppBarConfiguration);
    NavigationUI.setupWithNavController(navigationView, navController);
}

Logcat with error:

2022-06-13 19:13:12.720 9197-9197/com.example.llm E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.llm, PID: 9197
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.llm/com.example.llm.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
    at com.example.llm.MainActivity.onCreate(MainActivity.java:63)
    at android.app.Activity.performCreate(Activity.java:7994)
    at android.app.Activity.performCreate(Activity.java:7978)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:223) 
    at android.app.ActivityThread.main(ActivityThread.java:7656) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

I have done some research but every page does the same procedure: take the view from .xml, take the data to an ArrayList or some similar object, create the adapter and do a .setAdapter.

I don’t really know if this would be enough information, if it isn’t I can add everything needed.

activity_main.xml:

    <?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout 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_id="@+id/drawer_layout"
    android_layout_width="match_parent"
    android_layout_height="match_parent"
    android_fitsSystemWindows="true"
    tools_openDrawer="start">

    <include
        android_id="@+id/app_bar_main"
        layout="@layout/app_bar_main"
        android_layout_width="match_parent"
        android_layout_height="match_parent" />

    <com.google.android.material.navigation.NavigationView
        android_id="@+id/nav_view"
        android_layout_width="wrap_content"
        android_layout_height="match_parent"
        android_layout_gravity="start"
        android_fitsSystemWindows="true"
        app_headerLayout="@layout/nav_header_main"
        app_menu="@menu/activity_main_drawer" />
    </androidx.drawerlayout.widget.DrawerLayout>

By the way, the view where the SearchView is used is the following:

    <?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">


    <ImageView
        android_id="@+id/imLogo"
        android_layout_width="250dp"
        android_layout_height="100dp"
        android_layout_marginTop="36dp"
        android_src="@drawable/logo"
        app_layout_constraintEnd_toEndOf="parent"
        app_layout_constraintStart_toStartOf="parent"
        app_layout_constraintTop_toTopOf="parent" />

    <TextView
        android_id="@+id/txtNombre"
        android_layout_width="wrap_content"
        android_layout_height="wrap_content"
        android_text="Grupo: "
        android_textSize="16sp"
        app_layout_constraintBottom_toBottomOf="@+id/spinner"
        app_layout_constraintEnd_toStartOf="@+id/spinner"
        app_layout_constraintHorizontal_bias="0.917"
        app_layout_constraintStart_toStartOf="parent"
        app_layout_constraintTop_toTopOf="@+id/spinner"
        app_layout_constraintVertical_bias="0.5" />

    <Spinner
        android_id="@+id/spinner"
        android_layout_width="210dp"
        android_layout_height="48dp"
        android_contentDescription="@string/spinnerRef"
        app_layout_constraintBottom_toTopOf="@+id/txtDescription"
        app_layout_constraintEnd_toEndOf="parent"
        app_layout_constraintHorizontal_bias="0.5"
        app_layout_constraintStart_toEndOf="@+id/txtNombre"
        app_layout_constraintStart_toStartOf="parent"
        app_layout_constraintTop_toBottomOf="@+id/imLogo" />

    <TextView
        android_id="@+id/txtDescription"
        android_layout_width="wrap_content"
        android_layout_height="wrap_content"
        android_layout_marginBottom="16dp"
        android_text="Búsqueda de alumnos: "
        android_textSize="16sp"
        app_layout_constraintBottom_toTopOf="@+id/relativeLayout3"
        app_layout_constraintEnd_toEndOf="parent"
        app_layout_constraintHorizontal_bias="0.172"
        app_layout_constraintStart_toStartOf="parent" />

    <RelativeLayout
        android_id="@+id/relativeLayout3"
        android_layout_width="613dp"
        android_layout_height="521dp"
        app_layout_constraintBottom_toBottomOf="parent"
        app_layout_constraintEnd_toEndOf="parent"
        app_layout_constraintStart_toStartOf="parent"
        app_layout_constraintTop_toBottomOf="@+id/imLogo"
        app_layout_constraintVertical_bias="0.38">

        <SearchView
            android_id="@+id/search"
            android_layout_width="fill_parent"
            android_layout_height="wrap_content"
            android_iconifiedByDefault="false">

            <requestFocus />
        </SearchView>

        <ListView
            android_id="@+id/listview"
            android_layout_width="fill_parent"
            android_layout_height="fill_parent"
            android_layout_below="@+id/search" />

    </RelativeLayout>

    <Button
        android_id="@+id/btnCrearCurso"
        android_layout_width="350dp"
        android_layout_height="60dp"
        android_backgroundTint="@color/blueFlag"
        android_text="Añadir Estudiantes"
        app_cornerRadius="20dp"
        app_iconPadding="0dp"
        app_layout_constraintBottom_toBottomOf="parent"
        app_layout_constraintEnd_toEndOf="parent"
        app_layout_constraintStart_toStartOf="parent"
        app_layout_constraintTop_toBottomOf="@+id/relativeLayout3" />

</androidx.constraintlayout.widget.ConstraintLayout>