Failed to build in android studio. Can't find why it's not working

at first when I started the app the error sad "unable to delete directory" so i delete that file. The virtual devices started but the app keep stopping and I can't find why.

    Process: com.example.library, PID: 10926      java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.library/com.example.library.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)          at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)          at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)          at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)          at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)          at android.os.Handler.dispatchMessage(Handler.java:106)          at android.os.Looper.loop(Looper.java:223)          at android.app.ActivityThread.main(ActivityThread.java:7656)          at java.lang.reflect.Method.invoke(Native Method)          at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)       Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference          at com.example.library.MainActivity.onCreate(MainActivity.java:20)          at android.app.Activity.performCreate(Activity.java:8000)          at android.app.Activity.performCreate(Activity.java:7984)          at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)          at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)          at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)           at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)           at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)           at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)           at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)           at android.os.Handler.dispatchMessage(Handler.java:106)           at android.os.Looper.loop(Looper.java:223)           at android.app.ActivityThread.main(ActivityThread.java:7656)           at java.lang.reflect.Method.invoke(Native Method)           at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)  

this is the error when i start the app

https://stackoverflow.com/questions/66961521/failed-to-build-in-android-studio-cant-find-why-its-not-working April 06, 2021 at 09:06AM