[Test Scenario Editor] Improve Test Scenario creation UX

Requirement of this Enhancement is to improve the UX experience for the user when a new Test Scenario is created.

Currently the Popup which appears when accessing a new (empty) created scesim file, permits the user to close the popup (pressing X or "Cancel" button) in an inconsistent state. Moreover, if the user select on DMN dropdown an invalid item (es. "Select") the button "Create" remains enabled.