Android Studio 按钮不显示

Question

我需要在大学使用 android studio，直到现在一切正常，但现在我无法在我的活动中显示按钮等内容。我尝试重新启动并使缓存无效，但它在模拟器或我的物理手机上没有显示任何内容。

那是第一个活动的 xml：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">


    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="158dp"
        android:layout_marginTop="185dp"
        android:text="Button"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

那是第一个活动的代码

package com.example.blatt4.Aufgabe1



import android.content.Intent
import android.os.Bundle
import android.os.PersistableBundle
import android.util.Log
import android.view.MotionEvent
import android.widget.Button
import androidx.appcompat.app.AppCompatActivity
import com.example.blatt4.MainActivity
import com.example.blatt4.R


class FirstActivity:AppCompatActivity() {
    val TAG = "FirstActivity"


    override fun onCreate(savedInstanceState: Bundle?, persistentState: PersistableBundle?) {
        super.onCreate(savedInstanceState, persistentState)
        setContentView(R.layout.activity_first)

        val button = findViewById<Button>(R.id.button2)


        button.setOnClickListener{
            startActivity(Intent(this, MainActivity::class.java))
        }
    }

    override fun onTouchEvent(event: MotionEvent?): Boolean {

        when (event!!.actionMasked) {
            MotionEvent.ACTION_DOWN -> Log.d(TAG, "Touch event started at x = " + event.x + ", y = " + event.y)
            MotionEvent.ACTION_UP -> Log.d(TAG, "User lifted their finger at x = " + event.x + ", y = " + event.y);
            MotionEvent.ACTION_MOVE -> Log.d(TAG, "User is moving their finger at x = " + event.x + ", y = " + event.y)


            MotionEvent.ACTION_CANCEL -> Log.d(TAG, "User canceled the touch event at x = " + event.x + ", y = " + event.y);
            MotionEvent.ACTION_BUTTON_PRESS -> Log.d(TAG, "Button pressed at x = " + event.x + ", y = " + event.y);


            //Multi Touch
            /**
            MotionEvent.ACTION_POINTER_DOWN -> Log.d(TAG, "Pointer Down" + event.x + ", y=" + event.y);
            MotionEvent.ACTION_POINTER_UP -> Log.d(TAG, "Pointer Up" + event.x + ", y=" + event.y);
            */
        }


        return true
    }

}

在我的主要 Activity 中，它工作正常，但之后没有任何显示。

package com.example.blatt4

import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import com.example.blatt4.Aufgabe1.FirstActivity
import com.example.blatt4.Aufgabe2.SecondActivity
import com.example.blatt4.Aufgabe3.ThirdActivity
import com.example.blatt4.Aufgabe4.FourthActivity

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val aufgabe1 = findViewById<Button>(R.id.aufgabe1)
        val aufgabe2 = findViewById<Button>(R.id.aufgabe2)
        val aufgabe3 = findViewById<Button>(R.id.aufgabe3)
        val aufgabe4 = findViewById<Button>(R.id.aufgabe4)

        aufgabe1.setOnClickListener {
            startActivity(Intent(this, FirstActivity::class.java))
        }
        aufgabe2.setOnClickListener {
            startActivity(Intent(this, SecondActivity::class.java))
        }
        aufgabe3.setOnClickListener {
            startActivity(Intent(this, ThirdActivity::class.java))
        }
        aufgabe4.setOnClickListener {
            startActivity(Intent(this, FourthActivity::class.java))
        }

    }
}

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">

    <Button
        android:id="@+id/aufgabe1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="150dp"
        android:layout_marginTop="170dp"
        android:text="Aufgabe1"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/aufgabe2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="150dp"
        android:layout_marginTop="53dp"
        android:text="Aufgabe2"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/aufgabe1" />

    <Button
        android:id="@+id/aufgabe3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="150dp"
        android:layout_marginTop="53dp"
        android:text="Aufgabe3"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/aufgabe2" />

    <Button
        android:id="@+id/aufgabe4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="150dp"
        android:layout_marginTop="53dp"
        android:text="Aufgabe4"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/aufgabe3" />

</androidx.constraintlayout.widget.ConstraintLayout>

我尝试使缓存无效，同步 gradle，但在主活动之后我没有在屏幕上显示任何内容。