Android checkbox check for null to avoid app crash

I am very newbie on android studio and java. I have made a simple page which contain check boxes, and a button to print out in a toast message the checked box selected. The problem is if i clicked on submit button without selecting anything,the app stop working and crash. What i want to achieve is if i didn't select anything and i click on submit button it should send me a toast message "Please select a box" without making the app crash.

XML code:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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"
android:orientation="vertical"
android:padding="16dp"
tools:context=".MainActivity">
<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textStyle="bold"
    android:text="Favorite Courses: "
    android:textSize="20sp"
    android:layout_marginTop="30dp"
    android:layout_below="@+id/txDate"
    android:id="@+id/fvCourses"/>
<CheckBox
    android:id="@+id/cbCs"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_toRightOf="@+id/fvCourses"
    android:layout_alignBottom="@+id/fvCourses"
    android:text="Computer Science" />
<CheckBox
    android:id="@+id/cbAcc"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_toRightOf="@+id/fvCourses"
    android:layout_below="@+id/cbCs"
    android:text="Accounting" />
<CheckBox
    android:id="@+id/cbGraphic"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_toRightOf="@+id/fvCourses"
    android:layout_below="@+id/cbAcc"
    android:text="Graphic Designer" />
<Button
    android:id="@+id/btnapply"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/radioGroup"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="330dp"
    android:text="Confirm" />
    </RelativeLayout>

Java code:

package test.cb.app;

import androidx.appcompat.app.AppCompatActivity;

import android.widget.TextView;
import android.widget.Toast;
import android.widget.Button;
import android.widget.CheckBox;
public class MainActivity extends AppCompatActivity {

     TextView textView;
    CheckBox cbCs,cbAcc,cbGraphic;

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

        cbCs = findViewById(R.id.cbCs);
        cbAcc = findViewById(R.id.cbAcc);
        cbGraphic = findViewById(R.id.cbGraphic);

        Button btnapply = findViewById(R.id.btnapply);
        btnapply.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                StringBuffer result = new StringBuffer();
                result.append("Computer Science check : ").append(cbCs.isChecked());
                result.append("\nAccounting check : ").append(cbAcc.isChecked());
                result.append("\nGraphic check :").append(cbGraphic.isChecked());
                Toast.makeText(MainActivity.this, result.toString(),
                        Toast.LENGTH_LONG).show();
            }
        });
        }

            }

I expect that i have to check for null values on the submit button.

What would be the code to achieve that ?

Any help would be greatly appriciated.

Thank you!