Why I don't get error and application doesn't open - android-studio

Question

Why Android Studio doesn't detect error but when I try to open the app on mobile device it doesn't open.
It only opens if I comment these 2 lines in the Activity but of course I don't get full functionality. Am I missing something?
Commented lines
spinner.findViewById(R.id.spinnerCountries);
spinner.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_spinner_dropdown_item,CountryData.countryNames));
*Activity.java
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.Spinner;
import com.google.firebase.FirebaseException;
import com.google.firebase.auth.FirebaseAuth;
public class RegisterActivity extends AppCompatActivity {
private Spinner spinner;
private EditText editText;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_register);
spinner.findViewById(R.id.spinnerCountries);
spinner.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_spinner_dropdown_item,CountryData.countryNames));
editText = findViewById(R.id.editTextPhone);
findViewById(R.id.buttonContinue).setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
String code = CountryData.countryAreaCodes[spinner.getSelectedItemPosition()];
String number = editText.getText().toString().trim();
if (number.isEmpty() || number.length() < 10){
editText.setError("Valid number is required");
editText.requestFocus();
return;
}
String phonenumber = "+" + code + number;
Intent intent = new Intent (RegisterActivity.this,OtpActivity.class);
intent.putExtra("phonenumber",phonenumber);
startActivity(intent);
}
});
}
#Override
protected void onStart() {
super.onStart();
if (FirebaseAuth.getInstance().getCurrentUser() !=null) {
Intent intent = new Intent (this, MainActivity.class);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
startActivity(intent);
}
}
}
This is the file which I am trying to connect to the Activity
CountryData.java
public class CountryData {
public static final String[] countryNames =
{"Afghanistan", "Albania",
"Algeria", "Andorra", "Angola", "Antarctica", "Argentina",
"Armenia", "Aruba", "Australia", "Austria", "Azerbaijan",
"Bahrain", "Bangladesh", "Belarus", "Belgium", "Belize", "Benin",
"Bhutan", "Bolivia", "Bosnia And Herzegovina", "Botswana",
"Brazil", "Brunei Darussalam", "Bulgaria", "Burkina Faso",
"Myanmar", "Burundi", "Cambodia", "Cameroon", "Canada",
"Cape Verde", "Central African Republic", "Chad", "Chile", "China",
"Christmas Island", "Cocos (keeling) Islands", "Colombia",
"Comoros", "Congo", "Cook Islands", "Costa Rica", "Croatia",
"Cuba", "Cyprus", "Czech Republic", "Denmark", "Djibouti",
"Timor-leste", "Ecuador", "Egypt", "El Salvador",
"Equatorial Guinea", "Eritrea", "Estonia", "Ethiopia",
"Falkland Islands (malvinas)", "Faroe Islands", "Fiji", "Finland",
"France", "French Polynesia", "Gabon", "Gambia", "Georgia",
"Germany", "Ghana", "Gibraltar", "Greece", "Greenland",
"Guatemala", "Guinea", "Guinea-bissau", "Guyana", "Haiti",
"Honduras", "Hong Kong", "Hungary", "India", "Indonesia", "Iran",
"Iraq", "Ireland", "Isle Of Man", "Israel", "Italy", "Ivory Coast",
"Jamaica", "Japan", "Jordan", "Kazakhstan", "Kenya", "Kiribati",
"Kuwait", "Kyrgyzstan", "Laos", "Latvia", "Lebanon", "Lesotho",
"Liberia", "Libya", "Liechtenstein", "Lithuania", "Luxembourg",
"Macao", "Macedonia", "Madagascar", "Malawi", "Malaysia",
"Maldives", "Mali", "Malta", "Marshall Islands", "Mauritania",
"Mauritius", "Mayotte", "Mexico", "Micronesia", "Moldova",
"Monaco", "Mongolia", "Montenegro", "Morocco", "Mozambique",
"Namibia", "Nauru", "Nepal", "Netherlands", "New Caledonia",
"New Zealand", "Nicaragua", "Niger", "Nigeria", "Niue", "Korea",
"Norway", "Oman", "Pakistan", "Palau", "Panama",
"Papua New Guinea", "Paraguay", "Peru", "Philippines", "Pitcairn",
"Poland", "Portugal", "Puerto Rico", "Qatar", "Romania",
"Russian Federation", "Rwanda", "Saint Barthélemy", "Samoa",
"San Marino", "Sao Tome And Principe", "Saudi Arabia", "Senegal",
"Serbia", "Seychelles", "Sierra Leone", "Singapore", "Slovakia",
"Slovenia", "Solomon Islands", "Somalia", "South Africa",
"Korea, Republic Of", "Spain", "Sri Lanka", "Saint Helena",
"Saint Pierre And Miquelon", "Sudan", "Suriname", "Swaziland",
"Sweden", "Switzerland", "Syrian Arab Republic", "Taiwan",
"Tajikistan", "Tanzania", "Thailand", "Togo", "Tokelau", "Tonga",
"Tunisia", "Turkey", "Turkmenistan", "Tuvalu",
"United Arab Emirates", "Uganda", "United Kingdom", "Ukraine",
"Uruguay", "United States", "Uzbekistan", "Vanuatu",
"Holy See (vatican City State)", "Venezuela", "Viet Nam",
"Wallis And Futuna", "Yemen", "Zambia", "Zimbabwe"};
public static final String[] countryAreaCodes = {"93", "355", "213",
"376", "244", "672", "54", "374", "297", "61", "43", "994", "973",
"880", "375", "32", "501", "229", "975", "591", "387", "267", "55",
"673", "359", "226", "95", "257", "855", "237", "1", "238", "236",
"235", "56", "86", "61", "61", "57", "269", "242", "682", "506",
"385", "53", "357", "420", "45", "253", "670", "593", "20", "503",
"240", "291", "372", "251", "500", "298", "679", "358", "33",
"689", "241", "220", "995", "49", "233", "350", "30", "299", "502",
"224", "245", "592", "509", "504", "852", "36", "91", "62", "98",
"964", "353", "44", "972", "39", "225", "1876", "81", "962", "7",
"254", "686", "965", "996", "856", "371", "961", "266", "231",
"218", "423", "370", "352", "853", "389", "261", "265", "60",
"960", "223", "356", "692", "222", "230", "262", "52", "691",
"373", "377", "976", "382", "212", "258", "264", "674", "977",
"31", "687", "64", "505", "227", "234", "683", "850", "47", "968",
"92", "680", "507", "675", "595", "51", "63", "870", "48", "351",
"1", "974", "40", "7", "250", "590", "685", "378", "239", "966",
"221", "381", "248", "232", "65", "421", "386", "677", "252", "27",
"82", "34", "94", "290", "508", "249", "597", "268", "46", "41",
"963", "886", "992", "255", "66", "228", "690", "676", "216", "90",
"993", "688", "971", "256", "44", "380", "598", "1", "998", "678",
"39", "58", "84", "681", "967", "260", "263"};
}