How to reproduce:
1. Create a new project and connect it to perforce
2. Make sure the "Overwrite Failed Checkout Assets" option in Project Settings > Editor is unchecked
2. Add and Submit the "SampleScene" from the Assets > Scenes folder
3. Open P4V with the same Workspace as used in the Editor
4. Find the "SampleScene.unity" file and in the right-click menu select "Change Filetype"
5. Add the Exclusive Checkout option (+l) and press OK
6. Submit "SampleScene.unity" file
7. Open a different Workspace in P4V
8. Get the latest version of "SampleScene.unity" file
9. Checkout "SampleScene.unity" and "SampleScene.unity.meta" files
10. Get back to Unity Editor
11. Make a change to the "SampleScene"
12. Save the Scene

Expected result: both of the windows are visible as shown in "ExpectedResult.png"
Actual result: the "Version Control Check Out Failed" window is completely black as shown in "BlackDialog.png"

Reproducible with: 2018.4.22f1, 2019.3.12f1, 2020.1.0a23
Could not test with: 2017.4.40f1 ( "Overwrite Failed Checkout Assets" option doesn't exist and files get overwritten automatically), 2020.1.0a24 and later ("The open scene(s) have been modified externally" dialog is missing)

Notes:
-This doesn't reproduce 100% of the time as shown in "Inconsistent.mp4"
-This can sometimes be reproduced even if the file doesn't have the "Exclusive Checkout" option marked. This happens when the filed is Checked Out Remotely and is Out Of Synch at the same time as shown in "NotLocked.png"