How can I fetch data to display on a new HTML page after clicking a button on a main index.html page?

I have two html files (index.html and allUsers.html) and a JavaScript index.js
The index.html has a button allUsersButton.

My goal is that when the allUsersButton is clicked, I should see the allUsers.html page, and be able to see the fetched data which should be injected into the div named allUsersDivId.

So far, no data loads on the allUsers.html page and I get an error on the console “Uncaught TypeError: Cannot set properties of null (setting ‘onclick’)”.
Should both the index.html and the allUsers.html have the script linked in them? What’s the best way to put this together?

index.html

<body>
    <form action="http://localhost:7050/hello" method="POST">
        <label for="username">User name:</label>
        <input type="text" id="username" name="username"><br><br>      
        <input type="submit" value="Submit">
    </form>

    <button type="button" id="allUsersButton">All Users</button>
    
    <script src="index.js"></script>
</body>

allUsers.html

<body>
    <div id = "allUsersDivId">
    </div>
    <script src="index.js"></script>
</body>

index.js
Here I have a function for fetching and inserting the data into the div allUsersDivId which is in the allUsers.html, and an onClick listening on the allUsersButton which is in the index.html.

document.getElementById("allUsersButton").onclick = function() {displayAllUsers()};

function displayAllUsers() {
    window.location.href='allUsers.html'
    fetch("http://localhost:7050/allusers")
    .then((response) => {
        if (response.ok) {
        return response.json();
        } else {
        throw new Error("NETWORK RESPONSE ERROR");
        }
    })
    .then(data => {
        for(var i = 0; i < data.length; i++){
           const userName = data[i].username 
           const userNameDiv = document.getElementById("allUsersDivId")
        
           const heading = document.createElement("h1")
           heading.innerHTML = userName
           userNameDiv.appendChild(heading)
        }
    })
.catch((error) => console.error("FETCH ERROR:", error));    
}

What’s the best way to link this all together?