QtCreator / QtDesigner crashes after ActiveX is inserted and the ui file is saved

I created a new project and used the edit pane in QtCreator to insert an ActiveX from another company "CSTSoft" (http://www.cstsoft.com/ they are providing knob, graphs... for industrial applications and you can download a trial version).
I open the ui file from QtCreator and define the AxWidget with the ActiveX is fine. I can compile and run the application.
BUT once I closed the ui file, it is not possible to reopen it (>>QTCreator AppCrash) unless I remove the property containing the ActiveX ID opening the ui file with the text editor.
Additionally I tried opening the saved ui file with QtDesigner instead of the edit pane in QtCreator and QtDesigner crashes too.
I have tried it randomly with other ActiveX available in the AxWidget define property. Some of them work, I can reopen the ui file without crash. Others (from other manufacturer's than CSTSoft too) don't work. Could it be the ActiveX itself? (My knowledge of ActiveX is weak, I used to consider them as black boxes).