E/AndroidRuntime: FATAL EXCEPTION: main Process: mateuszszufladowicz.pl, PID: 31885 java.lang.RuntimeException: Unable to start activity ComponentInfo{mateuszszufladowicz.pl/mateuszszufladowicz.pl.register}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2955) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3030) at android.app.ActivityThread.-wrap11(Unknown Source:0) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) at android.os.Handler.dispatchMessage(Handler.java:105) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6938) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at mateuszszufladowicz.pl.register.onCreate(register.java:15) at android.app.Activity.performCreate(Activity.java:7174) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2908) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3030)  at android.app.ActivityThread.-wrap11(Unknown Source:0)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)  at android.os.Handler.dispatchMessage(Handler.java:105)  at android.os.Looper.loop(Looper.java:164)  at android.app.ActivityThread.main(ActivityThread.java:6938)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)  Application terminated.