I have frequent error in android xml file under menu folder. Error parsing XML:unbound prefix -

i have error in menu_main.xml file under menu folder:

error:error parsing xml unbound prefix. 

my code is

<menu xmlns:android="http://schemas.android.com/apk/res/android">  <item     android:id="@+id/action_settings"     android:title="@string/action_settings"     android:orderincategory="100"     app:showasaction="never"/>  <item     android:id="@+id/menu_search"     android:title="@string/menu_search"     appcompat:showasaction="always"/> 

<?xml version="1.0" encoding="utf-8"?> <menu xmlns:android="http://schemas.android.com/apk/res/android" >  <item     android:id="@+id/action_settings"     android:title="@string/action_settings"     android:orderincategory="100"     android:showasaction="never"/>  <item     android:id="@+id/menu_search"     android:title="@string/menu_search"     android:showasaction="always"/>  </menu>