Check for updates window does not appear on startup even if Check for updates checkbox is selected.

To repro:
0. Precondition: in Editor Help -> Check for updates check checkbox Check for updates.
1. Open the Editor.

Actual result: Although I can see API request is sent once per hour or so, Check for updates window doesn't appear.

Expected result: Check for updates window should appear when the update is available.
Extended version of the expected result:
1. On first start up CheckForUpdate window should appear on startup of Editor if checkbox Check on update is set to True in registry. After that it’s being checked one a week.
2. If it’s the first installation of Unity - we set it to true.
3. If it’s not set - we show nothing.

Reproducible with: 2019.4.15f1, 2020.1.4f1, 2020.1.9f1
Not reproducible with: 2018.4.27f1, 2020.1.3f1
Could not test with: 2020.2.0b6, 2020.1.0a2 (alpha and beta versions are always reported as up to date)

Notes:
-Sometimes the Unity Editor Update Check window opens on startup but doesn't load and remains black. This generates a lot of errors