Estou usando o FirebaseAuth para registro de usuário com email e senha e já adicionei o plug-in e as dependências no meu projeto.

MainActivity.java

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
EditText ed_email, ed_pass;
Button but_login;
ProgressDialog progressDialog;
FirebaseAuth firebaseAuth;
Context context;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    context=getApplicationContext();
    FirebaseApp.initializeApp(context);
    firebaseAuth=FirebaseAuth.getInstance();

    ed_email= (EditText) findViewById(R.id.ed_email);
    ed_pass= (EditText) findViewById(R.id.ed_pass);
    but_login= (Button) findViewById(R.id.but_login);
    but_login.setOnClickListener(this);
    progressDialog=new ProgressDialog(this);
}
public void registerUser(){
    String email=ed_email.getText().toString().trim();
    String pass=ed_pass.getText().toString().trim();
    if(TextUtils.isEmpty(email)){
        Toast.makeText(getApplicationContext(),"Invalid Input",Toast.LENGTH_SHORT).show();
        return;
    }
    if(TextUtils.isEmpty(pass)){
        Toast.makeText(getApplicationContext(),"Invalid Input",Toast.LENGTH_SHORT).show();
        return;
    }
    progressDialog.setMessage("You are registering...");
    progressDialog.show();
    firebaseAuth.createUserWithEmailAndPassword(email,pass).addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
        @Override
        public void onComplete(@NonNull Task<AuthResult> task) {
            if (task.isSuccessful()) {
                Toast.makeText(getApplicationContext(), "Success", Toast.LENGTH_SHORT).show();
                progressDialog.hide();
            } else {
                Toast.makeText(getApplicationContext(), "Sorry...!!!", Toast.LENGTH_SHORT).show();
                progressDialog.hide();
            }
        }

    });


}

@Override
public void onClick(View v) {
    registerUser();
}

}

logcat -

com.skapsdevelopment.firebase E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.skapsdevelopment.firebase, PID: 31993
Theme: themes:{default=overlay:com.resurrectionremix.pitchblack, iconPack:com.baranovgroup.nstyle, fontPkg:com.resurrectionremix.pitchblack, com.android.systemui=overlay:com.resurrectionremix.pitchblack, com.android.systemui.navbar=overlay:com.resurrectionremix.pitchblack}
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.skapsdevelopment.firebase/com.skapsdevelopment.firebase.MainActivity}: java.lang.IllegalStateException: Default FirebaseApp is not initialized in this process com.skapsdevelopment.firebase. Make sure to call FirebaseApp.initializeApp(Context) first.
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2450)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510)
   at android.app.ActivityThread.-wrap11(ActivityThread.java)
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)
   at android.os.Handler.dispatchMessage(Handler.java:102)
   at android.os.Looper.loop(Looper.java:148)
   at android.app.ActivityThread.main(ActivityThread.java:5461)
   at java.lang.reflect.Method.invoke(Native Method)
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.IllegalStateException: Default FirebaseApp is not initialized in this process com.skapsdevelopment.firebase. Make sure to call FirebaseApp.initializeApp(Context) first.
   at com.google.firebase.FirebaseApp.getInstance(Unknown Source)
   at com.google.firebase.auth.FirebaseAuth.getInstance(Unknown Source)
   at com.skapsdevelopment.firebase.MainActivity.onCreate(MainActivity.java:35)
   at android.app.Activity.performCreate(Activity.java:6251)
   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2403)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510) 
   at android.app.ActivityThread.-wrap11(ActivityThread.java) 
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363) 
   at android.os.Handler.dispatchMessage(Handler.java:102) 
   at android.os.Looper.loop(Looper.java:148) 
   at android.app.ActivityThread.main(ActivityThread.java:5461) 
   at java.lang.reflect.Method.invoke(Native Method) 
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

Meu aplicativo não está iniciando e mostra o seguinte erro:

java.lang.IllegalStateException: Default FirebaseApp is not initialized in this 
process com.skapsdevelopment.firebase. 
Make sure to call FirebaseApp.initializeApp(Context) first.

Por que o aplicativo não está sendo iniciado corretamente?