title: Button is not disabling even it is in valid condition
body: <p>In my chat app, I'm adding Phone verification from user. So I want that when user will enter the number, after that sendVerification button should be <code>setEnabled(true)</code> and I set <code>setEnabled(false)</code> as default, but when I run the app, it is still showing as enabled without entering the digits.</p>
  
  <p><strong>activity_phone_auth.xml</strong></p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      tools:context=".PhoneAuthActivity"&gt;
  
      &lt;include
          android:id="@+id/PhoneToolbar"
          layout="@layout/app_bar"&gt;
      &lt;/include&gt;
  
      &lt;LinearLayout
          android:id="@+id/DialLayout"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:layout_below="@id/PhoneToolbar"
          android:orientation="horizontal"
          android:weightSum="10"&gt;
  
          &lt;ImageView
              android:id="@+id/dial"
              android:layout_width="50dp"
              android:layout_height="wrap_content"
              android:padding="10dp"
              android:src="@drawable/dial"
              android:layout_weight="1"/&gt;
  
          &lt;EditText
              android:id="@+id/PhoneNumber"
              android:layout_width="270dp"
              android:layout_height="wrap_content"
              android:hint="Phone Number"
              android:layout_weight="8"
              android:ems="10"
              android:inputType="phone"/&gt;
  
          &lt;ProgressBar
              android:id="@+id/PhoneProgress"
              style="?android:attr/progressBarStyle"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_weight="1" /&gt;
      &lt;/LinearLayout&gt;
  
      &lt;LinearLayout
          android:id="@+id/LockLayout"
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:layout_below="@id/DialLayout"
          android:orientation="horizontal"
          android:weightSum="10"&gt;
  
          &lt;ImageView
              android:id="@+id/lock"
              android:layout_width="50dp"
              android:layout_height="wrap_content"
              android:padding="10dp"
              android:src="@drawable/lock"
              android:layout_weight="1"/&gt;
  
          &lt;EditText
              android:id="@+id/code"
              android:layout_width="270dp"
              android:layout_height="wrap_content"
              android:hint="Verification Code"
              android:layout_weight="8"/&gt;
  
          &lt;ProgressBar
              android:id="@+id/CodeProgress"
              style="?android:attr/progressBarStyle"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:layout_weight="1" /&gt;
  
      &lt;/LinearLayout&gt;
  
      &lt;TextView
          android:id="@+id/VerificationText"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_alignParentBottom="true"
          android:layout_centerHorizontal="true"
          android:layout_marginBottom="91dp"
          android:text="A verification code will be sent to your phone number" /&gt;
  
      &lt;Button
          android:id="@+id/sendVerification"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_alignParentBottom="true"
          android:layout_centerHorizontal="true"
          android:layout_marginBottom="23dp"
          android:backgroundTint="#FF0000"
          android:text="Send Verification" /&gt;
  
  &lt;/RelativeLayout&gt;
  </code></pre>
  
  <p><strong>PhoneAuthActivity.java</strong></p>
  
  <pre><code>package com.jimmytrivedi.lapitchat;
  
  import android.os.Bundle;
  import android.support.v7.app.AppCompatActivity;
  import android.support.v7.widget.Toolbar;
  import android.view.View;
  import android.widget.Button;
  import android.widget.EditText;
  import android.widget.LinearLayout;
  import android.widget.ProgressBar;
  
  import com.google.firebase.FirebaseException;
  import com.google.firebase.auth.PhoneAuthCredential;
  import com.google.firebase.auth.PhoneAuthProvider;
  
  import java.util.concurrent.TimeUnit;
  
  public class PhoneAuthActivity extends AppCompatActivity {
  
      private LinearLayout DialLayout, LockLayout;
      private EditText PhoneNumber, code;
      private ProgressBar PhoneProgress, CodeProgress;
      private Button sendVerification;
      private Toolbar PhoneToolbar;
      private PhoneAuthProvider.OnVerificationStateChangedCallbacks mCallbacks;
      private String number;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_phone_auth);
  
          DialLayout = findViewById(R.id.DialLayout);
          LockLayout = findViewById(R.id.LockLayout);
          PhoneNumber = findViewById(R.id.PhoneNumber);
          code = findViewById(R.id.code);
          PhoneProgress = findViewById(R.id.PhoneProgress);
          CodeProgress = findViewById(R.id.CodeProgress);
          sendVerification = findViewById(R.id.sendVerification);
          PhoneToolbar = findViewById(R.id.PhoneToolbar);
  
          PhoneProgress.setVisibility(View.INVISIBLE);
          CodeProgress.setVisibility(View.INVISIBLE);
          sendVerification.setEnabled(false);
  
          setSupportActionBar(PhoneToolbar);
          getSupportActionBar().setTitle("Welcome to Phone Verification");
          getSupportActionBar().setDisplayHomeAsUpEnabled(true);
  
          number = PhoneNumber.getText().toString();
          if (number != null) {
              sendVerification.setEnabled(true);
          } else {
              sendVerification.setEnabled(false);
          }
  
          sendVerification.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
  
  
                  PhoneNumber.setEnabled(false);
                  PhoneProgress.setVisibility(View.VISIBLE);
  
  
                  PhoneAuthProvider.getInstance().verifyPhoneNumber(
                          number,
                          60,
                          TimeUnit.SECONDS,
                          PhoneAuthActivity.this,
                          mCallbacks
                  );
              }
          });
  
  
          mCallbacks = new PhoneAuthProvider.OnVerificationStateChangedCallbacks() {
              @Override
              public void onVerificationCompleted(PhoneAuthCredential phoneAuthCredential) {
  
              }
  
              @Override
              public void onVerificationFailed(FirebaseException e) {
  
              }
          };
      }
  }
  </code></pre>