hello world button in acumatica not working

here is my code. it adds a button to the customer screen, the button calls out but gets this in response,

0|<ctl00_phF_BAccount_t0_btnHelloWorld />

in AR303000 page aspx

<px:PXFormView ID="BAccount" runat="server" Width="100%" Caption="Customer Summary" DataMember="BAccount" NoteIndicator="True" FilesIndicator="True" ActivityIndicator="false" ActivityField="NoteActivity" LinkIndicator="true" BPEventsIndicator="true" DefaultControlID="edAcctCD">
<Template>
  <px:PXLayoutRule runat="server" StartColumn="True" ControlSize="XM" LabelsWidth="SM" ></px:PXLayoutRule>
    <px:PXButton runat="server" Text="Hello World" AutoCallBack="True" ID="btnHelloWorld">
 <AutoCallBack Command="HelloWorld" /></px:PXButton>
 <px:PXSegmentMask ID="edAcctCD" runat="server" DataField="AcctCD" FilterByAllFields="True" ></px:PXSegmentMask>
 <px:PXDropDown ID="edStatus" runat="server" DataField="Status" CommitChanges="True" ></px:PXDropDown>

in CODE in same customization project

using PX.Data;
using PX.Objects.AR; // for CustomerMaint graph
using PX.Objects.CR; // for BAccount DAC

public class CustomerMaint_Extension : PXGraphExtension<CustomerMaint>
{
    // Define the action matching the button we will create
    public PXAction<BAccount> HelloWorld;

    [PXButton] // Indicates this is a button action
    [PXUIField(DisplayName = "Hello World")] // Label for the button/action
    protected void helloWorld()
    {
        // If this code runs, it will throw an exception and show a popup error in the UI.
        throw new PXException("Hello world from the server!");
    }
}

why don't I get an error message returned?? my return is this, 0|<ctl00_phF_BAccount_t0_btnHelloWorld />

It sends out a callback to the server but the return is not as expected as I do not think my code is being hit. Any suggestions. In the long run, I want to button to populate field values. But start here to just get the button functional.

You need to use Customer DAC in your action, as it is the main DAC in the CustomerMaint graph:

public PXAction<Customer> HelloWorld;

You don't need to modify the ASPX page at all, the framework will display the button using the code from your graph extension.