Android - FirebaseApp / Firebase-Initialization is not starting

My problem was the "Manifest Merger". If I use the

xmlns:tools="http://schemas.android.com/tools"

and

tools:node="replace"

in the Application Tag of the Manifest, the FirebaseApp will not be initialized!

If you use replace attr on tools:node it will replace your lower priority declaration with the annotated one (see more here). You should use merge or merge only attributes instead.

So what I've exactly done:

This doesn't work:

<application
            android:name="xxx"
            android:icon="@mipmap/ic_launcher"
            android:label="@string/application_name"
            android:theme="@style/Theme.TemplateStyle"
            tools:node="replace">

this works:

<application
            android:name="xxx"
            android:icon="@mipmap/ic_launcher"
            android:label="@string/application_name"
            android:theme="@style/Theme.TemplateStyle">
            <!-- tools:node="replace"-->

If you want to overwrite or replace some attributes, use tools:replace instead!