To reproduce:

1. Open the project, attached by the user (case_928424-New_Unity_Project.zip)
2. Open the "scene" scene
3. Select Main Camera in the Hierarchy
4. Press Click Me button in the Inspector
5. In the Save Dialog navigate to somewhere outside the Assets folder
6. Press Save
7. In the popup press OK
8. Press the Click Me button without closing the Save Dialog
9. Repeat 5 - 9

Expected: it should not be possible to open another Save File dialog if one is already open

Reproduced 5.6.2p2, 2017.1.0f2, 2017.2.0b2