QtCreator disable run button adding existing file

When i add an existing file to a project the run and debug buttons become and stay disabled (grayed) both on the main gui and by build menu, the only way to enabled them again is to force the creator to parse the .pro file again like adding a space to the .pro, remove a file or close and open again the creator.
Actually i'm using version 3.6.1 but this problem was present on previous version, can't remember exactly from wich one, and it's still present in creator 3.6.83 (4.0.0-rc1), the one with the flat style.

I made a short video for easier understanding:
https://youtu.be/0PA7rBhf7PM

The problem looks very similar to this one:
http://stackoverflow.com/questions/23999834/run-button-is-disabled-in-qt-creator

Forum post:
https://forum.qt.io/topic/66268/qtcreator-disable-run-button-adding-existing-file/3