It's this line :

@foreach(var category in (List<Category>)ViewData["CategoryList"])

But I don't understand how it can produce a NullReferenceException as the HTML Code hereafter shows that it retreived entities through CategoryList :

<form id="editor_pane" method="POST">
        <input type="list" list="Category" placeholder="Catégorie" name="Category">
        <datalist id="Category">
            <option value="NodeJS"></option>
            <option value="csharp"></option>
        </datalist>

        <input type="text" placeholder="Saisissez le titre de votre astuce/article ici" name="Title">
        
        <input type="text" placeholder="Description" name="Description">
</form>