package com.rntestproject;

import android.app.Application;

import com.facebook.react.ReactApplication;
import com.facebook.react.ReactNativeHost;
import com.facebook.react.ReactPackage;
import com.facebook.react.shell.MainReactPackage;
import com.facebook.soloader.SoLoader;

import com.reactnativenavigation.NavigationApplication;

import java.util.Arrays;
import java.util.List;

public class MainApplication extends NavigationApplication {

  // private final ReactNativeHost mReactNativeHost = new ReactNativeHost(this) {
  //   @Override
  //   public boolean getUseDeveloperSupport() {
  //     return BuildConfig.DEBUG;
  //   }

  //   @Override
  //   protected List<ReactPackage> getPackages() {
  //     return Arrays.<ReactPackage>asList(
  //         new MainReactPackage()
  //     );
  //   }

  //   @Override
  //   protected String getJSMainModuleName() {
  //     return "src/index.android";
  //   }

  // };

  // @Override
  // public ReactNativeHost getReactNativeHost() {
  //   return mReactNativeHost;
  // }

  // @Override
  // public void onCreate() {
  //   super.onCreate();
  //   SoLoader.init(this, /* native exopackage */ false);
  // }

  @Override
  public String getJSMainModuleName() {
    return "src/index.android";
  }

  @Override
  public boolean isDebug() {
    return BuildConfig.DEBUG;
  }

  @Override
  public List<ReactPackage> createAdditionalReactPackages() {
    return Arrays.<ReactPackage>asList();
  }
}

package com.rntestproject;

import android.app.Application;

import com.facebook.react.ReactApplication;
import com.facebook.react.ReactNativeHost;
import com.facebook.react.ReactPackage;
import com.facebook.react.shell.MainReactPackage;
import com.facebook.soloader.SoLoader;

import com.reactnativenavigation.NavigationApplication;

import java.util.Arrays;
import java.util.List;

public class MainApplication extends NavigationApplication {

  @Override
  public String getJSMainModuleName() {
    return "src/index.android";
  }

  @Override
  public boolean isDebug() {
    return BuildConfig.DEBUG;
  }

  @Override
  public List<ReactPackage> createAdditionalReactPackages() {
    return Arrays.<ReactPackage>asList();
  }
}

Delete unused code after adding the react-native-nagivation