Android Studio Uygulama Durduruldu Hatası

E/AndroidRuntime: FATAL EXCEPTION: main.

Process: com.example.blogkisiselgelisimbakim, PID: 7510.

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.blogkisiselgelisimbakim/com.example.blogkisiselgelisimbakim.MainActivity}: java.lang.NullPointerException.

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2429)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2493)

at android.app.ActivityThread.access$800(ActivityThread.java:166)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1283)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:136)

at android.app.ActivityThread.main(ActivityThread.java:5584)

at java.lang.reflect.Method.invokeNative(Native Method)

at java.lang.reflect.Method.invoke(Method.java:515)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)

at dalvik.system.NativeStart.main(Native Method)

Caused by: java.lang.NullPointerException.

at com.example.blogkisiselgelisimbakim.MainActivity.onCreate(MainActivity.java:51)

at android.app.Activity.performCreate(Activity.java:5447)

at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094)

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2393)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2493)

at android.app.ActivityThread.access$800(ActivityThread.java:166)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1283)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:136)

at android.app.ActivityThread.main(ActivityThread.java:5584)

at java.lang.reflect.Method.invokeNative(Native Method)

at java.lang.reflect.Method.invoke(Method.java:515)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)

at dalvik.system.NativeStart.main(Native Method)

E/dalvikvm: Could not find class 'android.app.NotificationChannel', referenced from method com.onesignal.NotificationChannelManager.createChannel