对话框由于setcontentview而崩溃

Question

目标是要有一个菜单（位于menu_main xml上），一旦您单击“详细信息”，便会打开一个对话框（dialogDetails），该对话框代表一个详细信息表单，其中包括编辑文本，日期选择器和性别选择对话框。 请说，如果需要，我将上传xml文件（这是android studio中的java文件...） 非常感谢您的帮助：）

package com.example.android.work8;
import android.app.AlertDialog;
import android.app.DatePickerDialog;
import android.app.Dialog;
import android.content.ClipData;
import android.content.DialogInterface;
import android.content.SharedPreferences;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.RadioGroup;
import android.widget.Toast;

import java.util.Calendar;

public class MainActivity extends AppCompatActivity implements View.OnClickListener, RadioGroup.OnCheckedChangeListener {
    Dialog dialogGender, dialogDetails;
    Button btnSaveGender;
    Button btnSave, btnBirthDate, btnGender;
    EditText etFirstName, etLastName, etCity;
    RadioGroup rg1;
    SharedPreferences sp;

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

        etFirstName = (EditText) findViewById(R.id.etFirstName);
        etLastName = (EditText) findViewById(R.id.etLastName);
        etCity = (EditText) findViewById(R.id.etCity);

        lastSave();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        super.onCreateOptionsMenu(menu);
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        super.onOptionsItemSelected(item);
        int id = item.getItemId();

        if (id == R.id.myDetails) {
            createDetailsDialog();

        } else if (id == R.id.delete) {

        }
        return true;
    }

    public void createGenderDialog() {
        dialogGender = new Dialog(this);
        dialogGender.setContentView(R.layout.custom_gender);
        dialogGender.setTitle("Select your gender");
        dialogGender.setCancelable(false);
        btnSaveGender = (Button) dialogGender.findViewById(R.id.btnSaveGender);
        btnSaveGender.setOnClickListener(this);
        rg1 = (RadioGroup) dialogGender.findViewById(R.id.rg1);
        rg1.setOnCheckedChangeListener(this);

        dialogGender.show();
    }

        public void createDetailsDialog(){
            dialogDetails = new Dialog(this);
            dialogDetails.setContentView(R.layout.my_details);
            dialogDetails.setTitle("Insert details");
            dialogDetails.setCancelable(false);
            btnSave=(Button)dialogDetails.findViewById(R.id.btnSave);
            btnSave.setOnClickListener(this);
            btnBirthDate=(Button)dialogDetails.findViewById(R.id.btnBirthDate);
            btnBirthDate.setOnClickListener(this);
            btnGender=(Button)dialogDetails.findViewById(R.id.btnGender);
            btnGender.setOnClickListener(this);

            dialogDetails.show();
        }

        @Override
        public void onClick (View v){
            if (btnGender == v) {
                createGenderDialog();
            } else if (btnBirthDate == v) {
                Calendar systemCalender = Calendar.getInstance();
                int year = systemCalender.get(Calendar.YEAR);
                int month = systemCalender.get(Calendar.MONTH);
                int day = systemCalender.get(Calendar.DAY_OF_MONTH);
                DatePickerDialog datePickerDialog = new DatePickerDialog(this, new SetDate(), year, month, day);
                datePickerDialog.show();

            } else if (btnSave == v) {
                AlertDialog.Builder builder = new AlertDialog.Builder(this);
                builder.setTitle("Save details");
                builder.setMessage("do you want to save?");
                builder.setCancelable(true);
                builder.setPositiveButton("yes", new HandleAlertDialogListener());
                builder.setNegativeButton("No", new HandleAlertDialogListener());
                AlertDialog dialog = builder.create();
                dialog.show();

            } else if (btnSaveGender == v) {
                dialogGender.dismiss();
            }
        }

        @Override
        public void onCheckedChanged (RadioGroup group,int checkedId){
            if (group == rg1) {
                if (checkedId == R.id.rMale) {
                    btnGender.setText("male");
                } else if (checkedId == R.id.rFemale) {
                    btnGender.setText("female");
                }
            }
        }

        public class HandleAlertDialogListener implements DialogInterface.OnClickListener {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                if (which == -1) saveDetails();
                else
                    Toast.makeText(MainActivity.this, "Details were not saved", Toast.LENGTH_LONG).show();
                dialogDetails.dismiss();
            }
        }

    public class SetDate implements DatePickerDialog.OnDateSetListener {

        @Override
        public void onDateSet(DatePicker view, int year, int monthOfYear, int dayOfMonth) {
            monthOfYear = monthOfYear + 1;

            String str = dayOfMonth + "/" + monthOfYear + "/" + year;
            btnBirthDate.setText(str);
        }
    }

    public void saveDetails () {

        Toast.makeText(MainActivity.this, "Details were saved", Toast.LENGTH_LONG).show();

        SharedPreferences sp = getSharedPreferences("details1", 0);
        SharedPreferences.Editor editor = sp.edit();
        editor.putString("fname", etFirstName.getText().toString()).putString("lname", etLastName.getText().toString());
        editor.putString("city", etCity.getText().toString());
        editor.putString("gender", btnGender.getText().toString());
        editor.putString("bitrth", btnBirthDate.getText().toString());
        editor.commit();
    }

    public void lastSave () {
        SharedPreferences sp = getSharedPreferences("details1", 0);
        String strfname = sp.getString("fname", null);
        String strlname = sp.getString("lname", null);
        String strcity = sp.getString("city", null);
        String strgender = sp.getString("gender", null);
        String strbirth = sp.getString("bitrth", null);

        if (strfname != null)
            etFirstName.setText(strfname);
        else if (strlname != null)
            etLastName.setText(strlname);
        else if (strcity != null)
            etCity.setText(strcity);
        else if (strgender != null)
            btnGender.setText(strgender);
        else if (strbirth != null)
            btnBirthDate.setText(strbirth);
    }
}
//error message
E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.android.work8, PID: 13392
              java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
                  at com.example.android.work8.MainActivity.saveDetails(MainActivity.java:162)
                  at com.example.android.work8.MainActivity$HandleAlertDialogListener.onClick(MainActivity.java:138)
                  at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:167)
                  at android.os.Handler.dispatchMessage(Handler.java:111)
                  at android.os.Looper.loop(Looper.java:194)
                  at android.app.ActivityThread.main(ActivityThread.java:5691)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:372)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)