title: PhoneAuthProvider.getInstance().verifyPhoneNumber on a null object reference
body: <pre><code>public class OTPFragment extends Fragment {   
  private FirebaseAuth mAuth;
  PhoneAuthProvider.OnVerificationStateChangedCallbacks mCallbacks;
  PhoneAuthProvider.ForceResendingToken mResendToken;
  private String mVerificationId = null;
  </code></pre>
  
  <p>initializing Firebase and instance in oncreate</p>
  
  <pre><code>@Override
  public void onCreate(@Nullable Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      context = this.getActivity();
      FirebaseApp.initializeApp(context);
      mAuth = FirebaseAuth.getInstance();
      initFirebaseCallback();
  }
  </code></pre>
  
  <p>here got the error in PhoneAuthProvider.getInstance</p>
  
  <pre><code> private void sendOtp(String phoneNumber) {
      PhoneAuthProvider.getInstance().verifyPhoneNumber(
              "+91"+mobileNumber,         // Phone number to verify
              60,               // Timeout duration
              TimeUnit.SECONDS,   // Unit of timeout
              getActivity(),// Activity (for callback binding)
              mCallbacks);        // OnVerificationStateChangedCallbacks
  }
  </code></pre>
  
  <p>callback and everything is initialized before calling .verifyphonenumber
  Got error with this code,the log is</p>
  
  <pre><code>   java.lang.NullPointerException: Attempt to invoke virtual method 'void 
     com.google.firebase.auth.FirebaseAuth.zza(java.lang.String, long, java.util.concurrent.TimeUnit, 
     com.google.firebase.auth.PhoneAuthProvider$OnVerificationStateChangedCallbacks, 
     android.app.Activity, java.util.concurrent.Executor, boolean, java.lang.String)' on a null object 
     reference
  </code></pre>
  
  <p>callback method</p>
  
  <pre><code>  private void initFirebaseCallback() {
      mCallbacks = new PhoneAuthProvider.OnVerificationStateChangedCallbacks() 
     {
          @Override
          public void onVerificationCompleted(PhoneAuthCredential credential) {
  
          }
  
          @Override
          public void onVerificationFailed(FirebaseException e) {
  
              } else if (e instanceof FirebaseTooManyRequestsException) {
                  //error
              }
  
          }
  
          @Override
          public void onCodeSent(@NonNull String verificationId,
                                 @NonNull PhoneAuthProvider.ForceResendingToken 
  token) {
  
              mVerificationId = verificationId;
              mResendToken = token;
          }
      };
  }
  </code></pre>