NavigationView nav_draw = (NavigationView) findViewById(R.id.nav_view);
   Menu menu =  nav_draw.getMenu();
   MenuItem item =  menu.findItem(R.id.nav_myflat);
   1) Spinner spinner = (Spinner) item.getActionView();

   2) View myView = getLayoutInflater().inflate(R.layout.activity_main, null);
   View v = myView.findViewById(R.id.spinner);
   Spinner spinner = (Spinner) v;

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:tools="http://schemas.android.com/tools"
                android:layout_width="match_parent"
                android:layout_height="match_parent" xmlns:app="http://schemas.android.com/apk/res-auto"
                tools:context=".MainActivity">
        <android.support.v7.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:background="?attr/colorPrimary"
                android:elevation="4dp"
                android:minHeight="?attr/actionBarSize"
                android:paddingTop="@dimen/tool_bar_top_padding"
                android:transitionName="actionBar" />


        <android.support.v4.widget.DrawerLayout
                xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:tools="http://schemas.android.com/tools"
                android:id="@+id/drawer_layout"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                tools:openDrawer="start">

            <FrameLayout
                    android:id="@+id/fragment_container"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent"
                    android:orientation="vertical"
            />

            <android.support.design.widget.NavigationView
                    android:id="@+id/nav_view"
                    android:layout_width="wrap_content"
                    android:layout_height="match_parent"
                    android:layout_gravity="start"
                    app:itemIconTint="@color/colorAccent"
                    app:itemTextColor="@color/colorPrimaryDark"
                    app:menu="@menu/main_menu"
                      />
    </android.support.v4.widget.DrawerLayout>
</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
        <group
                android:id="@+id/group1"
                android:checkableBehavior="single">
               <item android:id="@+id/nav_uprav"
                  android:title="@string/drawer_item_uprav" >
                <menu>
                    <item android:id="@+id/nav_news"
                          android:title="@string/drawer_item_news" />
                    <item android:id="@+id/nav_obiav"
                          android:title="@string/drawer_item_obiav" />
                    <item android:id="@+id/submenu3"
                          android:title="SubMenu 3" />
                </menu>
            </item>
            <item android:id="@+id/nav_myflat"
                  android:title="@string/drawer_item_myflat" >
                  app:actionLayout="@layout/spinner"/>
                <menu>
                    <item android:id="@+id/nav_auth"
                          android:title="@string/drawer_item_auth" />
                    <item android:id="@+id/nav_video"
                          android:title="@string/drawer_item_video" />
                    <item android:id="@+id/submenu5"
                          android:title="SubMenu 6" />
                </menu>
            </item>
        </group>
</menu>

<?xml version="1.0" encoding="utf-8"?>
<Spinner xmlns:android="http://schemas.android.com/apk/res/android"
         android:id="@+id/spinner"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:layout_gravity="center_vertical"
         android:gravity="center_vertical" />