Hello Luc Baeten,
Thank you for using our services.
Ajax control need the page object not to be null. However in edit mode the page actually does not exist and you will get this error message. You can go about this error the same way as we do it in our controls. In CreateChildControls or in InitializeControls check if the page exists and then create the child controls, else just add a simple label saying that the control cannot be viewed in edit mode:
Label noDesignModeLabel =
"The control cannot be viewed in design mode"
//add other logic here
So in your case you should add the control to the controls collection only if the page is not null.
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