recieving "java.lang.NullPointerException: error while geting list of files in android

Question

I have problem when I want to get my fonts form my dataBase.

This is my Activity :

public class Arrange_fur_secondaryActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle saveInstanceState) {
        super.onCreate(saveInstanceState);
        setContentView(R.layout.activity_warning);

        String path = Environment.getExternalStorageDirectory().toString()+"/database";
        Log.d("FilesFilesFiles", "PathPathPath: " + path);
        File directory = new File(path);
        File[] files = directory.listFiles();
        Log.d("FilesFilesFilesFiles", "SizeSizeSizeSize: "+ files.length);
        for (int i = 0; i < files.length; i++)
        {
            Log.d("Files", "FileName:" + files[i].getName());
        }
    }
}

And My Logcat Error :

**019-11-12 21:36:25.504 24221-24221/sungjusin.gatag3 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: sungjusin.gatag3, PID: 24221
    java.lang.RuntimeException: Unable to start activity ComponentInfo{sungjusin.gatag3/sungjusin.gatag3.Arrange_fur_secondaryActivity}: java.lang.NullPointerException: Attempt to get length of null array
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to get length of null array
        at sungjusin.gatag3.Arrange_fur_secondaryActivity.onCreate(Arrange_fur_secondaryActivity.java:34)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
2019-11-12 21:36:25.527 24221-24221/sungjusin.gatag3 I/Process: Sending signal. PID: 24221 SIG: 9**