Android webview javascript interface being removed on webforms partial postback

I am trying to interact with a WebForms site via a WebView inside an Android application.

I need to include a javascript interface to be able to send messages to the webview. However whenever a page does a partial postback, the interface gets removed and I get the javascript error "MyWebHost is undefined" when trying to access any of the functionality

Web Client:

public class MyWebviewClient extends WebViewClient
{

private View BusySpinner;
public String BaseUrl;

public MyWebviewClient(View busySpinner, String baseUrl)
{
    BusySpinner = busySpinner;
    BaseUrl = baseUrl;
}



@Override
public void onPageStarted(WebView view, String url, android.graphics.Bitmap favicon) {
    super.onPageStarted(view, url, favicon);
    BusySpinner.setVisibility(View.VISIBLE);
}

@Override
public void onPageFinished(WebView view, String url) {
    super.onPageFinished(view, url);
    if(url==null) { return;}

    if(url.contains("token_capture"))
    {
        view.loadUrl(BaseUrl);
        return;
    }
    
    view.addJavascriptInterface(new WebAppInterface(view.getContext()), "MyWebHost");
    BusySpinner.setVisibility(View.GONE);
}

 public class WebAppInterface 
 {

    public Context Context;

    public  WebAppInterface(Context context)
    {
        Context = context;
    }

    @JavascriptInterface
    public void HandleButtonPress()
    {
        Toast.makeText(Context, "The user pressed the button in the web view", Toast.LENGTH_LONG);
    }
    
}
}

sample interaction

<!-- works after page load, buy not after a partial postback -->
<div onclick="MyWebHost.HandleButtonPress();"> Click Me</div>