Hello I stumpled to this exact same error and discovered after 2 hours debuggin where the error is happening.

If you have namespace Ui

you must have in header:

Private:
Ui::MyClass *ui

Then in CPP you need to have #Include "MyClass.h" #Include "ui_MyClass.h" and in the code you must have:

ui(new Ui::MyClass) in the constructor statement.

Then only the Creator finds the ui file.

My problem was that I had several Namespaces like:

namespace MyApp { namespace core { namespace gui { class MyClass

When using some other namespace than Ui the creator cannot turn Ui::MyClass to pruple meaning that it indicates that code works. SO I changed it to MyApp::core::gui::MyClass *ui

This did break the Creator finding slot where Ui is. Changed that line in header back to Ui::MyClass *ui and then slots worked in Designer again. How ever for some reason the creator editor cannot understand the Ui::MyClass *ui because the namespace Ui does not exist. This may be a creator bug.

Hope this helps.