findFragmentById returns null value - android-studio

Question

I created 2 fragments a detailFrag and listFrag but when I'm trying to hide/show a particular fragment using fragment manager the findFragmentById(R.id.detailFrag) and findFragmentById(R.id.listFrag) returns a null value.
I searched for similar problems but none of the solutions worked for me .
Here I'm pasting the code of my MainActivity file where the error is being generated:
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tvDescription=findViewById(R.id.tvDescription);
descriptions = getResources().getStringArray(R.array.descriptions);
if(findViewById(R.id.layout_portrait) !=null)
{
FragmentManager manager=this.getSupportFragmentManager();
manager.beginTransaction()
.hide(manager.findFragmentById(R.id.detailFrag))
.show(manager.findFragmentById(R.id.listFrag))
.commit();
}
if(findViewById(R.id.layout_land)!=null)
{
FragmentManager manager = this.getSupportFragmentManager();
manager.beginTransaction()
.show(manager.findFragmentById(R.id.detailFrag))
.show(manager.findFragmentById(R.id.listFrag))
.commit();
}
}
Here the FragmentManager class I have used is of Androidx.fragment.app because I'm unable to import the android.app.support.v4.Fragment
I have tried implementation 'com.android.support:support-v4:28.0.0' in build.gradle file but it shows an error.
I was supposed to use the support library v4 here but due to that problem I'm unable to use it,not sure if that might have caused the problem.
XML Files:
MainActivity(portrait)
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="horizontal"
android:id="#+id/layout_portrait"
tools:context=".MainActivity">
<fragment
android:id="#+id/listFrag"
android:name="com.example.fragmentspractice.ListFrag"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="1"
tools:layout="#layout/fragment_list" />
<fragment
android:id="#+id/detailFrag"
android:name="com.example.fragmentspractice.DetailFrag"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="2"
tools:layout="#layout/fragment_detail" />
</LinearLayout>
MainActivity(Landscape):
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="horizontal"
android:id="#+id/layout_land"
tools:context=".MainActivity">
<fragment
android:id="#+id/listFrag"
android:name="com.example.fragmentspractice.ListFrag"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="1"
tools:layout="#layout/fragment_list" />
<fragment
android:id="#+id/detailFrag"
android:name="com.example.fragmentspractice.DetailFrag"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="2"
tools:layout="#layout/fragment_detail" />
listFrag:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/listFrag"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".ListFrag">
<ListView
android:id="#+id/lvList"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
detailFrag:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/detailFrag"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#color/colorPrimaryDark"
android:orientation="vertical"
tools:context=".DetailFrag">
<TextView
android:id="#+id/tvDescription"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginLeft="20dp"
android:layout_marginTop="20dp"
android:layout_marginRight="20dp"
android:text="TextView"
android:textColor="#FFFFFF"
android:textSize="18sp" />
</LinearLayout>