Apache POI WorkbookFatory.create() causes NullPointerException in Android Studio

Question

I have one class called DisplayResults that contains an array of strings called answers. I need to take this array, and use it to locate certain rows in an Excel spreadsheet. DisplayResults is an activity class, and ProductFinder is a non activity class (just a Java class). For some reason, I cannot get Apache POI to create a workbook without throwing a NullPointerException.

DisplayResults.java

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.ArrayList;

public class DisplayResults extends AppCompatActivity {

    ArrayList answers; // List of answers from the survey

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_results);


        Intent i = getIntent();
        answers = (ArrayList)i.getSerializableExtra("answers");


        ProductFinder p = new ProductFinder(answers,this);
    }


}

Once I declare the ProductFinder p object, I would like to use it to interact with an excel sheet.

ProductFinder.java

public class ProductFinder {


    ArrayList answers;

    ProductFinder(ArrayList answers, Context context)
    {
        this.answers = answers;
        InputStream inputStream = context.getResources().openRawResource(
                context.getResources().getIdentifier("product_list",
                        "raw", context.getPackageName()));

        Workbook wb = null;
        try {
            wb = WorkbookFactory.create(inputStream);

        } catch (IOException e)
        {
            System.err.print("error opening file");
        }


            System.out.println("Workbook has " + wb.getNumberOfSheets() + " Sheets ");


    }
}

I am getting a NullPointerException on the line where I call getNumberOfSheets(), and the stack trace points to that function as the problematic one. I made an if statement to check if the InputStream is null, and it is not. Also, my product_list.xlsx sheet is stored in the res/raw directory of my project.