Android App doesn't respond when clicking on the back-arrow button

Hello Fellow developers!
I have developed an Android App that uses the Flickr API to display any image upon users request. I am the final step of the development. Everything is looking great, except for when you click on any image to expand it in detail view & when you click the return arrow to go back - DOES NOT respond...
Any help would be greatly appreciated:)

ViewPhotoDetails.java:

package com.example.flickrbrowser;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ImageView;
import android.widget.TextView;
import com.squareup.picasso.Picasso;
import androidx.annotation.RequiresApi;
public class ViewPhotoDetailsActivity extends BaseActivity {
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.photo_details);
activateToolbarWithHomeEnabled();
Intent intent = getIntent();
Photo photo = (Photo) intent.getSerializableExtra(PHOTO_TRANSFER);
TextView photoTitle = (TextView) findViewById(R.id.photo_title);
photoTitle.setText("Title: " + photo.getTitle());
TextView photoTags = (TextView) findViewById(R.id.photo_tags);
photoTags.setText("Tags: " + photo.getTags());
TextView photoAuthor = (TextView) findViewById(R.id.photo_author);
photoAuthor.setText(photo.getAuthor());
ImageView photoImage = (ImageView) findViewById(R.id.photo_image);
Picasso.with(this).load(photo.getLink())
.error(R.drawable.placeholder)
.placeholder(R.drawable.placeholder)
.into(photoImage);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu_photo_details, menu);
return true;
}
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return false;
}
}    

photo_details.xml:

<?xml version="1.0" encoding="utf-8"?>
<ScrollView
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:flickr="http://schemas.android.com/apk/res-auto"
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"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.example.flickrbrowser.ViewPhotoDetailsActivity"
tools:showIn="@layout/photo_details">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<include
android:id="@+id/app_bar"
layout="@layout/toolbar"
android:layout_width="match_parent"
android:layout_height="?actionBarSize" />
<androidx.cardview.widget.CardView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginLeft="16dp"
android:layout_marginRight="16dp"
android:layout_marginTop="16dp"
flickr:cardBackgroundColor="?colorPrimary"
flickr:cardCornerRadius="8dp"
flickr:cardPreventCornerOverlap="false"
flickr:contentPaddingTop="16dp"
flickr:contentPaddingBottom="16dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<FrameLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="@+id/photo_image"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_marginBottom="8dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginTop="8dp"
android:scaleType="centerCrop"
android:src="@drawable/placeholder" />
<TextView
android:id="@+id/photo_author"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="top|center"
android:paddingTop="8dp"
android:textColor="@color/flickrSecondaryTextColor"
android:textSize="14sp" />
</FrameLayout>
<TextView
android:id="@+id/photo_title"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginTop="8dp"
android:textColor="@color/flickrPrimaryTextColor"
android:textSize="14sp" />
<TextView
android:id="@+id/photo_tags"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginTop="8dp"
android:textColor="@color/flickrPrimaryTextColor"
android:textSize="10sp" />
</LinearLayout>
</androidx.cardview.widget.CardView>
</LinearLayout>
</ScrollView>

Source Code on GitHub

App UI

Back button has id: android.R.id.home. So you can handle it as

if (item?.itemId == android.R.id.home) {
finish()
}

or just modify your

override fun onOptionsItemSelected(item: MenuItem?): Boolean {
if (item.itemId == R.id.action_settings) {
// TODO - handle 'settings' click
}
return super.onOptionsItemSelected(item)
}

instead of returning just true/false so home button will be handled by super method.

Just as @Boken said, the back (also called the "up") button is a menu button which has id android.R.id.home so when that button is clicked you can handle the click like so:

@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle item selection
switch (item.getItemId()) { 
//You can also use if-else statements instead of switch (switch is a better option)
case R.id.action_settings:
return true;
case android.R.id.home: //when the back button is clicked
// handle this action here
return true;
default:
return super.onOptionsItemSelected(item);
}
}

if you want to return to the previous screen you use onBackPressed() like so:

    case android.R.id.home: //when the back button is clicked
onBackPressed();
return true;

(More Direct Answer) Was not checking for that menu item. Had to change onOptionsItemSelected (in ViewPhotoDetailsActivity.java) to

public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
} else if (id == android.R.id.home) {
finish();
}
return false;
}