I am new to silverstripe and just tried to add a new page type which will allow me to fetch content from a remote page.

So, I added the following to mysite/code/RemoteContentPage.php

<?php
class RemoteContentPage extends Page {
  function getCMSFields() {
    $fields = parent::getCMSFields();
    $fields->replaceField('Content', new TextField('Content', 'URL to Remote Content')); // we abuse the content Field now to save the URL, we could do another DB Field, but since we already have Content ...
  }
  function Content() {
    $Text = 'hejsan paag';
    return $Text;
  }
}
class RemoteContentPage_Controller extends Page_Controller {
}
?>

Then I ran /dev/build?flush=1 and tried to create a new remote content page.

Here the backend crashed and I received an alert box that something went wrong (what was not visible in any apache log files).

After that I refreshed the backend by reloading my browser and now the backend page tree menu started to grow indefinitely, adding empty rows to itself and then performing a new request in a infinite loop.

When I removed RemoteContentPage.php from mysite/code and ran /dev/build again, remoteContentPage was still available in the /dev/build output and the backend crashes in the same way. Logout/Login from the backend did not help either.

Will clear all cookies and retry. What did I do wrong when trying to add a new page type?

The problem was that

getCMSFields() did not return any value.

Adding

return $fields;

at the end of the function made it all work again.