Could not find the rendering in the HTML loaded from server. Sitecore experience editor issue

Question

We recently moved from Sitecore 7.2 to 9.0.2 and since then we have started getting following error when we try to add a control(by clicking "Add here") with datasource from the experience editor. This is only happening on the 9.0.2 instance of Sitecore. Ours is not a MVC project. Just simple web forms. This happens on clicking either the first or the second Add here on the page. We are not referencing Prototype.js in the project so not sure if putting jQuery into No-conflict mode will help. Also, we are able to add controls with datasource by going to presentation details of the page in Sitecore. Any help is greatly appreciated.

What I have tried:

We are not referencing Prototype.js in the project so not sure if putting jQuery into No-conflict mode will help. Also, we are able to add controls with datasource by going to presentation details of the page in Sitecore. I also tried to create a new control(rendering) but it did not help. Adding/removing allowed controls of the placeholder also did not help. I tried removing all JS references from the layout one by one and it did not help sadly.

Below are the screenshots of the error I am getting.

Error in JS.

Answer 1

The issue was that in Sitecore 9, when a user adds a control on a page using the Experience Editor, on the final add button click (after providing a datasource) is treated as a POST event. In Sitecore 7.2, it was a GET request.

We had some code written in our backend (.cs file) which was looking to see if it a POST type of request and if yes, redirect to a different page. Like below.

if (Request.HttpMethod == "POST")
{
 //redirect to a different page
}

I added a check here to only run this code when user is NOT in Experience Editor mode like below.

if (Request.HttpMethod == "POST" && !Repository.IsUserEditting())
{
 //redirect to a different page
}

This fixed the issue.

Thanks. Let me know if anyone has any questions.

Answer 2

I had exactly the same error on Sitecore 10.2. We have a controller rendering that has two methods with the same name, one with [HttpGet] and other one with [HttpPost] attribute, like that:

public class FormController
{
    [HttpGet]
    public virtual ActionResult RenderForm()
    {
        ...
        return View("~/Views/Form/Form.cshtml", model);
    }

    [HttpPost]
    public virtual ActionResult RenderForm(FormCollection form)
    {
        ...
        return Redirect(redirectUrl);
    }
}

The Post method does redirect at the end and because Sitecore is using POST request when adding component there was a redirect on the server side.

I added a condition to the Post method like that:

[HttpPost]
public virtual ActionResult RenderForm(FormCollection form)
{
    if (Sitecore.Context.PageMode.IsExperienceEditorEditing)
    {
        return RenderForm();
    }

    ...
}