The dialog "Add Existing Files ..." opens from the context menu, when right-clicking on a project in the projects pane. The dialog window does not close. Neither clicking on "Cancel" nor on "Open" does anything. The only way to continue is to kill QtCreator and restart it.
Way to reproduce:
1) Create a new project.
2) Right-click on the project in the project pane.
3) Click on "Add Existing Files ...".