Unable to get provider com.google.firebase.provider.FirebaseInitProvider

1.

Add the applicationId to the application's build.gradle:

android {
    ...
    defaultConfig {
        applicationId "com.example.my.app"
        ...
    }
}

And than Clean Project -> Build or Rebuild Project

---

2. If your minSdkVersion <= 20 (https://developer.android.com/studio/build/multidex)

Use Multidex correctly.

application's build.gradle

android {
...               
    defaultConfig {
    ....
        multiDexEnabled true
    }
    ...
}

dependencies {
    implementation 'com.android.support:multidex:1.0.3'
    ...
}

manifest.xml

<application
    ...
    android:name="android.support.multidex.MultiDexApplication" >
    ...

---

3.

If you use a custom Application class

public class MyApplication extends MultiDexApplication {
    @Override
    protected void attachBaseContext(Context context) {
        super.attachBaseContext(context);
        MultiDex.install(this);
    }
}

manifest.xml

<application
    ...
    android:name="com.example.my.app.MyApplication" >
    ...

I was with the same problem in devices with SDK < 22, but for me the reason is the MultiDex, the MultiDex.install must be in the attachBaseContext method.

If you are using MultiDex, try this:

public class YourApplication extends Application {

    @Override
    protected void attachBaseContext(Context context) {
        super.attachBaseContext(context);
        MultiDex.install(this);
    }
    @Override
    public void onCreate() {
        super.onCreate();
        Realm.init(this); //initialize other plugins 

    }
}

app/build.gradle:

android {

    compileSdkVersion 24
    buildToolsVersion "24.0.2"

    defaultConfig {
        applicationId "com.test.app"
        minSdkVersion 16
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
        multiDexEnabled true

    }

    dexOptions {
        javaMaxHeapSize "4g"
    }

....
}

    dependencies {
        compile fileTree(dir: 'libs', include: ['*.jar'])

        compile 'com.android.support:appcompat-v7:24.2.1'
        ...
        compile 'com.google.firebase:firebase-messaging:9.6.1'
        compile 'com.google.android.gms:play-services:9.6.1'
        compile 'com.android.support:multidex:1.0.1'

    }
...

The accepted answer didn't solve my problem.

If you are using Multidex, your Application should extends MultiDexApplication instead of Application.

MyApplication.java

public class MyApplication extends MultiDexApplication{
     ...
}

AndroidManifest.xml

<application
      android:name="your.package.name.MyApplication"
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      ...
      />

Hope it helps.