How to solve the error in the AndroidManifest file "Unresolved class 'MainActivity' "?

My AndroidManifest file is showing an error "Unresolved class 'MainActivity' ", but i have a java class and file by that name in the package and it is supposed to be the first activity of the program so when i try to run the program it crashes up , i've checked my code but couldn't find any problem there .

java code for MainActivity file

package com.example.tictactoe;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.app.Activity;
import android.view.View;
import android.widget.Button;
import android.content.Intent;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    Button forward;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        forward = (Button)findViewById(R.id.start);
        forward.setOnClickListener(new View.OnClickListener()
                                    {
                                       @Override
                                       public void onClick(View v)
                                       {
                                           startActivity(new Intent(MainActivity.this,GameUi.class));
                                       }
                                    });
    }
    @Override
    public void onBackPressed() {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setCancelable(false);
        builder.setMessage("Do you want to Exit?");
        builder.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                finish();
            }
        });
        builder.setNegativeButton("No",new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                dialog.cancel();
            }
        });
        AlertDialog alert=builder.create();
        alert.show();
    }
}

java code for second activity which my program uses ("GameUi.java")

package com.example.tictactoe

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.view.View;
import android.widget.Button;
import android.os.Bundle;

public class GameUi extends AppCompatActivity {
    Button btn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_game_ui);
        btn=(Button)findViewById(R.id.backbtn);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                    startActivity(new Intent(GameUi.this, MainActivity.class));
            }
        });
    }
}

AndroidManifest file code

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.tictactoe">

    <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/Theme.TicTacToe">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".GameUi">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

MainActivity xml file 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="#E5E6AD"
        tools:context=".MainActivity">
    <TextView
        android:id="@+id/Heading"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_row="0"
        android:layout_column="0"
        android:layout_marginBottom="332dp"
        android:editable="false"
        android:text="Hello Aliens!"
        android:textAlignment="center"
        android:textColor="#000000"
        android:textSize="50dp"
        android:textStyle="bold|italic"
        app:layout_constraintBottom_toBottomOf="parent"
        tools:layout_editor_absoluteX="16dp" />

    <ImageView
        android:id="@+id/Alienimg"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="36dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@id/Heading"
        app:srcCompat="@drawable/ic_launcher_foreground" />
    
    <Button
        android:id="@+id/Startbtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="#000066"
        android:letterSpacing="0.5"
        android:padding="0dp"
        android:text="Start"
        android:textStyle="bold"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/Alienimg"
        app:layout_constraintVertical_bias="0.886" />

</androidx.constraintlayout.widget.ConstraintLayout>