at android.app.ActivityThread.main(ActivityThread.java:7073)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:964)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.myapplication2.MainActivity.onCreate(MainActivity.java:47)

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

       Button button = findViewById(R.id.button1);
       button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                openanmeldung();
            }
        });







        BottomNavigationView navigation = findViewById(R.id.bottomnav);
        navigation.setOnNavigationItemSelectedListener(this);


        getSupportFragmentManager().beginTransaction().replace(fragment_container, new Home_Screen()).commit();


    }

    private void openanmeldung(){
        Intent intent = new Intent(MainActivity.this,AnmeldungButton.class);
        startActivity(intent);
    }

 <android.support.v7.widget.CardView

            android:layout_width="match_parent"
            android:layout_height="173dp"
            app:cardCornerRadius="8dp"
            android:layout_marginTop="80dp"
            android:layout_marginLeft="30dp"
            android:layout_marginRight="30dp"
            >

        <Button
                android:text="Mehr infos"
                android:layout_width="120dp"
                android:layout_height="35dp"
                android:id="@+id/button1"
                android:layout_weight="1"
                android:layout_marginTop="130dp"
                android:layout_marginRight="170dp"

                />


        <LinearLayout android:layout_height="match_parent"
                      android:layout_width="match_parent"
                      android:background="@color/maincolor"

                >

        </LinearLayout>

        <TextView android:layout_height="wrap_content"
                  android:layout_width="wrap_content"
                  android:textColor="@color/white"
                  android:text="HPI - 29.06.19"
                  android:textSize="24dp"
                />

        <TextView android:layout_height="wrap_content"
                  android:layout_width="wrap_content"
                  android:textColor="@color/white"
                  android:text="Sitzung: H-E.51"
                  android:textSize="16dp"
                  android:layout_marginTop="40dp"

                />
        <TextView android:layout_height="wrap_content"
                  android:layout_width="wrap_content"
                  android:textColor="@color/white"
                  android:text="Juni22, 2019"
                  android:textSize="16dp"
                  android:layout_marginTop="40dp"
                  android:layout_marginRight="20dp"
                  android:layout_marginLeft="270dp"






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

package com.example.myapplication2;


import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;


public class AnmeldungButton extends Fragment {

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.moreinffos, container, false);
    }
}