Android : widget not showed inside widget list

I've develop an app, and now i would make a widget for do some tasks. I've follow some tutorials, but widget is not showed inside widget list. This is my code:

manifest:

<uses-sdk
    android:minSdkVersion="12"
    android:targetSdkVersion="19" />

<application

    ....

     <activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:windowSoftInputMode="stateHidden|adjustPan" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <receiver
        android:name="WidgetProvider"
        android:icon="@drawable/my_image"
        android:label="@string/widget_label" >
        <intent-filter>
            <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />

        </intent-filter>

        <meta-data
            android:name="android.apppwidget.provider"
            android:resource="@xml/widget_info" />
    </receiver>
</application>

this is widget_info.xml

<?xml version="1.0" encoding="utf-8"?>
<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
    android:initialLayout="@layout/widget_layout"
    android:minHeight="40dp"
    android:minWidth="40dp"
    android:resizeMode="horizontal|vertical" >

</appwidget-provider>

and widget_layout is a simple LinearLayout whith an imageView inside.

WidgetProvider.java for now is just an empty class.

When i install the app on my device, widget isn't showed inside widget list. What's wrong?