Why MainApplication.Java file is missing? for manually linking React-Native library how can i proceed?

I am customizing jitsi codebase according my requirement. I am building native sdk. I need a library for file sharing named react-native-file-picker.
In the installation process i need MainApplication.java file but i don’t see any MainApplication.java file in android folder.

// file: MainApplication.java
...

import com.filepicker.FilePickerPackage; // import package

public class MainApplication extends Application implements ReactApplication {

   /**
   * A list of packages used by the app. If the app uses additional views
   * or modules besides the default ones, add more packages here.
   */
    @Override
    protected List<ReactPackage> getPackages() {
        return Arrays.<ReactPackage>asList(
            new MainReactPackage(),
            new FilePickerPackage() // Add package
        );
    }
...
}

How can i manually link this type of library. Please help me someone