这是我的代码，你可以在我向onclick listener声明了id的类中看到，每个代码似乎都没有问题，只有0个错误，但当我运行应用程序时，它崩溃并说“尝试调用虚拟方法'void android.view.view.setonclicklistener（android.view.view$onclick）”，在使用它之前，我是否需要写下任何东西才能在mainactivity类中使用fragment元素？？任何捐款都是有益的。

package com.login.mobi.loginapp;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.FragmentManager;
import androidx.viewpager2.widget.ViewPager2;

import android.app.DatePickerDialog;
import android.os.Bundle;
import android.view.View;
import android.widget.DatePicker;
import android.widget.EditText;

import com.google.android.material.tabs.TabLayout;

import java.util.Calendar;

public class MainActivates extends AppCompatActivity {
EditText et1;

    TabLayout tabLayout;
    ViewPager2 pager2;
    FragmentAdapter adapter;
    Calendar calendar=Calendar.getInstance();
    final int year=calendar.get(Calendar.YEAR);
    final int month=calendar.get(Calendar.MONTH);
    final int day=calendar.get(Calendar.DAY_OF_MONTH);

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

EditText txt10=findViewById(R.id.txtnum10);
        tabLayout = findViewById(R.id.tab_layout);
        pager2 = findViewById(R.id.view_pager2);

        FragmentManager fm = getSupportFragmentManager();
        adapter = new FragmentAdapter(fm, getLifecycle());
        pager2.setAdapter(adapter);

        tabLayout.addTab(tabLayout.newTab().setText("Primary info"));
        tabLayout.addTab(tabLayout.newTab().setText("Next"));
        tabLayout.addTab(tabLayout.newTab().setText("Finish-up"));

        tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                pager2.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });
        et1=(EditText) findViewById(R.id.et1);
        et1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                DatePickerDialog datePickerDialog=new DatePickerDialog(
                        MainActivates.this, new DatePickerDialog.OnDateSetListener() {
                    @Override
                    public void onDateSet(DatePicker view, int year, int month, int day) {
                        month =month+1;
                        String date=day+"/"+month+"/"+year;
                        et1.setText(date);
                    }
                },year,month,day);
                datePickerDialog.show();
            }
        });

        pager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageSelected(int position) {
                tabLayout.selectTab(tabLayout.getTabAt(position));
            }
        });

    }
}