FloatingActionButton does not work

My FloatingActionButton does not work. In my application I have a fragment and two tabs. In one of the tabs, the floatingActionButton works, the other does not. This is the main fragment layout, where the button is located:

<!-- The navigation drawer -->
<ListView android:id="@+id/left_drawer"
    android:layout_width="240dp"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:choiceMode="singleChoice"
    android:divider="@android:color/transparent"
    android:dividerHeight="0dp"
    android:background="@color/actionbar"/>

<!-- The main content view -->
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/main_content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".WorkRequestOpenTabs">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/appbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/appbar_padding_top"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay"
            app:layout_scrollFlags="scroll|enterAlways">

        </android.support.v7.widget.Toolbar>

        <android.support.design.widget.TabLayout
            android:id="@+id/tabs"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:tabSelectedTextColor="@color/white"
            app:tabMode="scrollable"
            app:tabGravity="fill"/>

        <io.codetail.widget.RevealFrameLayout
            xmlns:android="http://schemas.android.com/apk/res/android"
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <Button android:id="@+id/btnTabAction"
                android:layout_height="0dp"
                android:layout_width="fill_parent"
                android:background="@color/colorAccent"
                android:text="@string/EndLabor"
                android:layout_alignParentBottom="true"
                android:visibility="invisible"/>

        </io.codetail.widget.RevealFrameLayout>

    </android.support.design.widget.AppBarLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@+id/appbar"
        app:layout_behavior="@string/appbar_scrolling_view_behavior" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignParentEnd="true"
        android:layout_alignParentRight="true"
        android:layout_gravity="end|bottom"
        android:layout_margin="@dimen/fab_margin"
        android:src="@android:drawable/ic_dialog_email" />

</RelativeLayout>

This is the Aba (photos) code, where I need to adjust an action for the button:

   @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    super.onCreateView(inflater, container, savedInstanceState);
    Log.i(this.getClass().getCanonicalName(), "###### onCreateView");
    rootView = inflater.inflate(R.layout.tabworkrequestdocuments, container, false);

    FloatingActionButton fab = (FloatingActionButton) getActivity().findViewById(R.id.fab);
    fab.bringToFront();

    if(fab != null) {

        fab.setImageResource(R.drawable.add_sign);
        fab.setVisibility(View.VISIBLE);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                addDocument();
            }
        });
    }

    return rootView;
}