在Android中，请确保首先调用FirebaseApp.initializeApp(Context)。

我遇到了这个问题，并在这个网站上看到了一些答案，但没有得到任何正确的解决方案。
\n我之前使用的是Firebase的旧版本，它工作正常，但是当我尝试使用升级和将Firebase类更新为DatabaseReference时，它显示错误并且无法工作。
\n我添加了我的清单文件的全部代码，请帮助我解决这个问题。
\n这是我的清单文件：
\n

\n下面是我的模块app代码：
\n

apply plugin: 'com.android.application'
android {
    compileSdkVersion 26
    buildToolsVersion "26.0.0"
    defaultConfig {
        applicationId "firebasechat.com.firebasechat"
        minSdkVersion 15
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        multiDexEnabled  true
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    packagingOptions {
        exclude 'META-INF/LICENSE'
        exclude 'META-INF/LICENSE-FIREBASE.txt'
        exclude 'META-INF/NOTICE'
    }
}
dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:26.+'
    compile 'com.android.volley:volley:1.0.0'
    compile "com.google.firebase:firebase-database:11.0.0"
    compile 'com.google.android.gms:play-services:11.0.0'
    compile 'com.android.support:recyclerview-v7:25.0.0'
    testCompile 'junit:junit:4.12'
}

\n和项目gradle：
\n

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.3'
        classpath 'com.google.gms:google-services:4.2.0'// Updated version of google service
    }
}
allprojects {
    repositories {
        jcenter()
        maven {
            url "https://maven.google.com" // Google's Maven repository
        }
    }
}
task clean(type: Delete) {
    delete rootProject.buildDir
}

\n下面是我的Activity：
\n

public class RegisterActivity extends AppCompatActivity {
    EditText username, password;
    Button registerButton;
    String user, pass;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register);
        username = (EditText)findViewById(R.id.username);
        password = (EditText)findViewById(R.id.password);
        registerButton = (Button)findViewById(R.id.registerButton);
        FirebaseApp.initializeApp(this);
        registerButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                user = username.getText().toString();
                pass = password.getText().toString();
                final ProgressDialog pd = new ProgressDialog(RegisterActivity.this);
                pd.setMessage("Loading...");
                pd.show();
                String url = "https://pure-coda-174710.firebaseio.com/users.json";
                StringRequest request = new StringRequest(Request.Method.GET, url, new Response.Listener(){
                    @Override
                    public void onResponse(String s) {
                        DatabaseReference reference = FirebaseDatabase.getInstance()
                                .getReferenceFromUrl("https://pure-coda-174710.firebaseio.com/users");
                        if(s.equals("null")) {
                            reference.child(user).child("password").setValue(pass);
                            Toast.makeText(RegisterActivity.this, "registration successful", Toast.LENGTH_LONG).show();
                        }
                        else {
                            try {
                                JSONObject obj = new JSONObject(s);
                                if (!obj.has(user)) {
                                    reference.child(user).child("password").setValue(pass);
                                    Toast.makeText(RegisterActivity.this, "registration successful", Toast.LENGTH_LONG).show();
                                } else {
                                    Toast.makeText(RegisterActivity.this, "username already exists", Toast.LENGTH_LONG).show();
                                }
                            } catch (JSONException e) {
                                e.printStackTrace();
                            }
                        }
                        pd.dismiss();
                    }
                },new Response.ErrorListener(){
                    @Override
                    public void onErrorResponse(VolleyError volleyError) {
                        System.out.println("" + volleyError );
                        pd.dismiss();
                    }
                });
                RequestQueue rQueue = Volley.newRequestQueue(RegisterActivity.this);
                rQueue.add(request);
            }
        });
    }
}