Why does Fragment not work correctly when the device is rotated?

Question

Java

When displaying a fragment on the screen after rotating the device, I get the following error:

E/InputEventReceiver: Exception dispatching input event.
E/MessageQueue-JNI: Exception in MessageQueue callback: handleReceiveCallback
E/MessageQueue-JNI: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                        at com.example.user.miniapppikflowtwo.FragmentTipTextView.setView(FragmentTipTextView.java:31)
                        at com.example.user.miniapppikflowtwo.MainActivity$2.onClick(MainActivity.java:158)

Fragment code:

import android.view.ViewGroup;
import android.widget.TextView;

public class FragmentTipTextView extends Fragment {

    TextView textView;
    String string;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_tip_tv, container,false);
        textView = view.findViewById(R.id.fragment_tip_text_view);

        return view;

    }

    public void setTextView(String string){
        this.string = string;
    }

    public void setView(){
        textView.setText(string);  //ОШИБКА ТУТ
    }

}

Activity:

package com.example.user.miniapppikflowtwo;

import android.animation.PropertyValuesHolder;
import android.app.DialogFragment;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.os.PersistableBundle;
import android.support.design.widget.FloatingActionButton;
import android.support.v4.app.FragmentManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;


import com.db.chart.animation.Animation;
import com.db.chart.listener.OnEntryClickListener;
import com.db.chart.model.LineSet;
import com.db.chart.tooltip.Tooltip;
import com.db.chart.util.Tools;
import com.db.chart.view.LineChartView;

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;

public class MainActivity extends AppCompatActivity implements Dialog2.OnFragmentListener {

    LineChartView chart;
    DBRecorder dataBase;
    Toolbar toolbar;
    DialogFragment dlg1, dlg2;

    FragmentManager fragmentManager;
    FragmentTipTextView fragmentTipTextView;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        fragmentManager = getSupportFragmentManager();
        fragmentTipTextView = new FragmentTipTextView();

        if(fragmentManager.findFragmentByTag("TodayFragment") == null){
                System.err.println("Fragment был null");
                fragmentManager.beginTransaction().add(R.id.tip_container, fragmentTipTextView, "TodayFragment").commit();
            } else {
                System.err.println("Fragment не был null");
                fragmentManager.beginTransaction().show(fragmentManager.findFragmentByTag("TodayFragment")).commit();
            }

        chart = findViewById(R.id.chart);
        dataBase = new DBRecorder(getApplicationContext());

        dlg1 = new Dialog1();
        dlg2 = new Dialog2();

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                dlg2.show(getFragmentManager(), "dlg2");
            }
        });

        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar); //поддержка Android < 5.0
        getSupportActionBar().setTitle("Моя пыльца");

        setGraph();
    }

    String[] stringDate; //строки с датами
    float[] values; // значения

    private void setGraph(){

        //Тут настраивается внешний вид графика. К проблеме отношения не имеет

        chart.setOnEntryClickListener(new OnEntryClickListener() {
            @Override
            public void onClick(int setIndex, int entryIndex, Rect rect) {
                Recorder recorder = arrayList.get(entryIndex);

                fragmentTipTextView.setTextView(recorder.getTip());

                fragmentTipTextView.setView(); //ОШИБКА В ЭТОЙ СТРОКЕ

            }
        });

        //-------Показываем график-------

        chart.show(anim); //Показывает график с анимацией
    }

 //Другие методы, не влияющие на fragment
    
}

Activity XML:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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"
    tools:context=".MainActivity">


    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        style="@style/Toolbar"/>

    <android.support.v7.widget.CardView
        android:id="@+id/cardView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/toolbar">

        <com.db.chart.view.LineChartView
            android:id="@+id/chart"
            android:layout_width="match_parent"
            android:layout_marginLeft="8dp"
            android:layout_marginRight="8dp"
            android:layout_marginTop="8dp"
            android:layout_marginVertical="8dp"
            android:layout_height="250dp">

        </com.db.chart.view.LineChartView>

    </android.support.v7.widget.CardView>

    <android.support.v7.widget.CardView
        android:id="@+id/tip_container"
        android:layout_width="match_parent"
        android:layout_height="120dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/cardView">

    </android.support.v7.widget.CardView>

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/ic_add_white_24dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginRight="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

</android.support.constraint.ConstraintLayout>

Fragment markup just in case:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout

    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/fragment_tip_text_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:textColor="@color/primaryTextColor"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

This is a very stupid problem, please help