To reproduce:

1. Open the project, attached by user (test.zip)
2. Open "test" scene if not open
3. In the top menu select Tools -> FileTest
4. After a pop up appears, press Cancel
5. In the Project browser, right click on empty space and press Refresh
6. Observe how assets disappear

Expected: pressing cancel in a pop-up does not remove all the assets in the Assets folder
Actual: all the assets are removed from the Assets folder if null is passed into the "to" parameter in CopyFileOrDirectory method

Reproduced in 5.5.0f3, 5.5.3p2, 5.6.0f3, 2017.1.0b5
Did not reproduce in 5.4.5p1
Regression introduced in 5.5.0f3

Fixed in 2017.2.0a4