Student Project - Fatal Error: Uncaught Error: Call to a member function prepare() on null in php

Question

I'm working on a student project. I'm building an e-commerce website to sell soaps, and the error message I'm getting is the following.

Fatal error: Uncaught Error: Call to a member function prepare() on null in /home/customer/www/myname.com/public_html/projects/call-of-the-river/objects/soap.php:41 Stack trace: #0 /home/customer/www/myname.com/public_html/projects/call-of-the-river/index.php(11): Soap->readAll() #1 {main} thrown in /home/customer/www/mattroddy.com/public_html/projects/call-of-the-river/objects/soap.php on line 41

Code from Index.php lines 1-17:

// Imports the database and the class files
include_once 'config/database.php';
include_once 'objects/soap.php';

// Creates a database object based on that class
$database = new Database();

// Creates a conn with the database object, and is stored in the var
$conn = $database->getConnection();
$sham = new Soap($conn);
$stmt = $sham->readAll();

// This will set up the page title string in a var that header.php will use:
$pageTitle = 'Home';

// Brings in the HTML for the top of the page
include_once('includes/header.php');

Code from Soap.php lines 33-46:

/**
 * This method reads all the row data from the table
 *
 * @return an object (like an array) full of row data
 */
function readAll()
{
    // Prepares a query using that connection, store it in a var $stmt
    $stmt = $this->conn->prepare('SELECT * FROM '.$this->tableName);
    // Runs the query, which causes $stmt to become full of the retrieved data.
    $stmt->execute();
    // Send the data back to where the function was called
    return $stmt;
}