title: phoneauthprovider's oncodesent not working
body: <p>So I have been trying to solve this problem but I can't seem to know what's wrong. I have a button that when clicked calls PhoneAuthProvider which has 4 options: OnVerifiicationCompleted, OnVerificationFailed, OnCodeSent, and onCodeAutoRetrieval. The problem is that oncodesent is being called but the code that I put in there doesn't work, not even a debug log. All I get is an SMS message showing a verification code. And when I set a string value to onCodeSent's verificationID, the string is value: null. Here's my code:</p>
  
  <pre><code>sendLink.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View v) {
              popSound.start();
  
              if (greenCheck.getVisibility() == View.VISIBLE) {
                  //send link
  
                  // we will finish this activity and send the link to the number
                  // an option to resend code to the number: it will be provided at the link
                  // in this format: resend code to (XXX)-XxX-XXXX
                  String number = phoneNumber.getText().toString();
                  phoneNumber.setText("");
                  sendLink.setEnabled(false);
  
                  PhoneAuthProvider.getInstance().verifyPhoneNumber(
                          number,
                          60,
                          TimeUnit.SECONDS,
                          MobileNumberActivity.this,
                          new PhoneAuthProvider.OnVerificationStateChangedCallbacks() {
                              @Override
                              public void onVerificationCompleted(PhoneAuthCredential phoneAuthCredential) {
                                  signInWithPhoneAuthCredential(phoneAuthCredential);
                              }
  
                              @Override
                              public void onVerificationFailed(FirebaseException e) {
  
                              }
  
                              @Override
                              public void onCodeSent(String s, PhoneAuthProvider.ForceResendingToken forceResendingToken) {
                                  super.onCodeSent(s, forceResendingToken);
                                  // verificationID never gets assigned s and it is null. log.d isn't on logcat
                                  verificationID = s;
                                  Log.d("testing", "onCodeSent: " + verificationID);
                              }
  
                              @Override
                              public void onCodeAutoRetrievalTimeOut(String s) {
                                  super.onCodeAutoRetrievalTimeOut(s);
                              }
                          }
                  );
  
  
                  Intent i = new Intent(MobileNumberActivity.this, VerificationActivity.class);
                  startActivity(i);
  
                  finish();
              } else if (phoneNumber.getText().toString().length() == 0) {
                  Toast.makeText(MobileNumberActivity.this, "Please enter a phone number", Toast.LENGTH_SHORT).show();
              } else {
                  Toast.makeText(MobileNumberActivity.this, "Please enter a valid phone number", Toast.LENGTH_SHORT).show();
              }
          }
      });
  </code></pre>