为什么我从github viewpage得到这个错误？

Question

我用这个教程：
https://github.com/neokree/MaterialTabs

并进入这一行：

this.setSupportActionBar(toolbar);

我收到此错误：

Caused by: java.lang.IllegalStateException: This Activity already has an action bar supplied by the window decor. Do not request Window.FEATURE_ACTION_BAR and set windowActionBar to false in your theme to use a Toolbar instead.
            at android.support.v7.app.ActionBarActivityDelegateBase.setSupportActionBar(ActionBarActivityDelegateBase.java:165)
            at android.support.v7.app.ActionBarActivity.setSupportActionBar(ActionBarActivity.java:92)
            at com.example.behzad.viewpage.MainActivity.onCreate(MainActivity.java:34)

我该如何解决？ TNX。