getActionBar().setTitle(); Java.lang.NullPoint异常解决方案

try changing your theme to this

<style name="AppTheme" parent="android:Theme.Holo.Light.DarkActionBar">

and not the AppCompat Version

It basically depends on whch api you are targeting.If it less than 14 use
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
and in your activity call your actionbar using this

getSupportActionBar().setDisplayHomeAsUpEnabled(true);