To reproduce:

1. Download attached project “New Unity Project (6).zip" and open in Unity
2. Open “SampleScene” scene
3. Select Tools > Test Window
4. Observe that in Editor opens a blank window
5. Enter to Play mode
6. Right-click on that blank window

Expected results: Everything works fine, no Error is printed
Actual results: In Console will be printed Error:

Menu CONTEXT/TEMPORARY-OBJECT-DISPLAY couldn't be found

Notes:
- This issue appears only on Windows
- On macOS, everything works fine
- Outside of Play mode, everything works as Expected

Reproduced on Unity 2018.3.0b7 and 2018.3.2f1
Not reproduced on Unity 2018.3.0b6, 2019.1.0a1 and 2019.1.0a12
Regression on Unity 2018.3.0b7