Get value of ASP:TextBox when clicking an ASP:Button - html

Question

I know this is a stupid question, and a simple question, but I feel like I'm going crazy as I can't get this working (even though I've done it a million times before).
I have a text box and a button in my ASP.NET page
<asp:TextBox ID="commentTextBox" inputtype="text" runat="server" CssClass="uk-textarea"></asp:TextBox>
<asp:Button ID="buttonComment" runat="server" CssClass="uk-button uk-button-text" text="Post Comment" OnClick="buttonComment_Click" />
These are a part of a larger set of HTML markup, that is wrapped in a:
<form id="form1" runat="server">
On button click, I do the following (get the value of the text box and post it to a database as a 'comment')
protected void buttonComment_Click(object sender, EventArgs e)
{
try
{
//string commentText = ((TextBox)FindControl("Comment")).Text;
string strDate = Request.Form["commentTextBox"].ToString();
Spark.Comment comment = new Spark.Comment
{
CommentByEmail = Context.User.Identity.Name.ToString(),
CommentDate = DateTime.Now,
SparkId = sparkId,
CommentText = strDate,
};
using (var client = new AmazonLambdaClient(Amazon.RegionEndpoint.APSoutheast2))
{
var request = new InvokeRequest
{
FunctionName = ConfigurationManager.AppSettings["lambdaArnPrefix"] + "lambdaSparkCreateComment",
Payload = JsonConvert.SerializeObject(comment),
InvocationType = InvocationType.RequestResponse
};
var response = client.Invoke(request);
}
Response.Redirect($"SparkDetail?action=comment");
}
catch (Exception ex)
{
sparkCardSingle.Text = ex.ToString();
}
}
I've tried all these variations of getting the value:
string commentText = ((TextBox)FindControl("commentTextBox")).Text
string commentText = Request.Form["commentTextBox"].ToString();
string commentText = CommentText.Text;
But no matter what, I get this error on post back
System.NullReferenceException: Object reference not set to an instance of an object. at APISpark.SparkDetail.Page_Load(Object sender, EventArgs e) in C:\Users\jmatson\Source\Repos\Production-Spark-Website\APISpark\SparkDetail.aspx.cs:line 136
I don't understand. I feel like I've done this code a billion times before and it works. I have no idea why it's not working now.