Not read content of file from assets folder not from activity: FileNotFoundException

Question

Android Studio 3.1. In my Manifest file:

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

MyApplication file:

public class MyApplication extends MultiDexApplication {

    @Override
    public void onCreate() {
        super.onCreate();
        init();
     }      

    private void init() {
        appContext = this;
    }

    public static Context getAppContext() {
        return appContext;
    }
}

In my FileUtil.java:

public static String getAssetsFileContent(Context context, String assetsFileName) {
        String content = "";
        try {
            InputStream is = context.getAssets().open(assetsFileName);
            int size = is.available();
            byte[] buffer = new byte[size];
            is.read(buffer);
            is.close();
            content = new String(buffer, "UTF-8");
        } catch (IOException e) {
            Debug.e(TAG, "getAssetsFileContent: " + e.getMessage(), e);
        }
        return content;
    }

In my custom class (not Activity)

@Override
            public void onError(ErrorResponse errorResponse) {             
                String dictionarySetDefaultContent = FileUtil.getAssetsFileContent(MyApplication.getAppContext(), "dictionary_set_default.json");
            }

In path myproject/app/src/main/res/assets

I save file dictionary_set_default.json

But I get error:

FileUtil(21086): getAssetsFileContent: dictionary_set_default.json
FileUtil(21086): java.io.FileNotFoundException: dictionary_set_default.json
FileUtil(21086):    at android.content.res.AssetManager.openAsset(Native Method)
FileUtil(21086):    at android.content.res.AssetManager.open(AssetManager.java:313)
FileUtil(21086):    at android.content.res.AssetManager.open(AssetManager.java:287)
FileUtil(21086):    at myproject.util.FileUtil.getAssetsFileContent(FileUtil.java:137)
FileUtil(21086):    at myproject.service.SyncService$1.onError(SyncService.java:51)
FileUtil(21086):    at myproject.api.DefaultRestClientCallback.onFailure(DefaultRestClientCallback.java:46)
FileUtil(21086):    at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$2.run(ExecutorCallAdapterFactory.java:79)
FileUtil(21086):    at android.os.Handler.handleCallback(Handler.java:739)
FileUtil(21086):    at android.os.Handler.dispatchMessage(Handler.java:95)
FileUtil(21086):    at android.os.Looper.loop(Looper.java:148)
FileUtil(21086):    at android.app.ActivityThread.main(ActivityThread.java:5417)
FileUtil(21086):    at java.lang.reflect.Method.invoke(Native Method)
FileUtil(21086):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
FileUtil(21086):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)