Login activity Android error

I was following this tutorial but when I finished I have an error.

Error log:

java.lang.NullPointerException
E/AndroidRuntime( 8128): at br.com.rodrigosilva.RegisterActivity.registerUser(RegisterActivity.java:181)
E/AndroidRuntime( 8128): at br.com.rodrigosilva.RegisterActivity.access$3(RegisterActivity.java:103)
E/AndroidRuntime( 8128): at br.com.rodrigosilva.RegisterActivity$1.onClick(RegisterActivity.java:77)
E/AndroidRuntime( 8128): at android.view.View.performClick(View.java:4456)
E/AndroidRuntime( 8128): at android.view.View$PerformClick.run(View.java:18465)
E/AndroidRuntime( 8128): at android.os.Handler.handleCallback(Handler.java:733)
E/AndroidRuntime( 8128): at android.os.Handler.dispatchMessage(Handler.java:95)
E/AndroidRuntime( 8128): at android.os.Looper.loop(Looper.java:136)
E/AndroidRuntime( 8128): at android.app.ActivityThread.main(ActivityThread.java:5086)
E/AndroidRuntime( 8128): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 8128): at java.lang.reflect.Method.invoke(Method.java:515)
E/AndroidRuntime( 8128): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
E/AndroidRuntime( 8128): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
E/AndroidRuntime( 8128): at dalvik.system.NativeStart.main(Native Method)
W/ActivityManager( 1071): Force finishing activity br.com.rodrigosilva/.RegisterActivity