Reproduction steps:
1. Download project "BugReport.zip" and open in Unity
2. Enter Play mode
3. Press on any number in Game view
4. Go to Scene view and observe red X (X.png)

Expected: in Game and Scene views to see an additive scene LevelToplistPopup
Actual: in Game view, there is no additive scene and in Scene view, there is red "X" marking place of the additive scene

Reproduced on: 2017.3.0a1; 2017.3.0b6; 2018.1.0a2
Not reproduced on: 5.6.4p1; 2017.1.2p2; 2017.2.0p1
Regression started: 2017.3.0a1

Note:
Works as expected if object's TallToplistAnchor child PopupFrame component's Aspect Ratio Fitter property Aspect Mode is changed to Envelope Parent and back to Fit in Parent;
Freezes on 2017.2.0a1-2017.2.0f3 and 2017.3.0a1-2017.3.0b4.