Reproduction steps:
1. Open attached project "adsProject"
2. Change the targeted platform to Android in the Build Settings
3. Open Assets > Scene
4. Enter Play Mode, click the "Load" button, and then the "Show" button
5. Close the ad and exit Play Mode
6. Enter Play Mode again, click the "Load" button, and then the "Show" button

Expected result: No errors are thrown and an ad is shown
Actual result: This error is thrown:
MissingReferenceException: The object of type 'GameObject' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.Advertisements.Placeholder.HideSkipButton (UnityEngine.GameObject canvasGameObject) (at Library/PackageCache/com.unity.ads@4.0.0/Runtime/Advertisement/Platform/Editor/Placeholder.cs:178)
UnityEngine.Advertisements.Placeholder.Show (System.String placementId, System.Boolean allowSkip) (at Library/PackageCache/com.unity.ads@4.0.0/Runtime/Advertisement/Platform/Editor/Placeholder.cs:79)
UnityEngine.Advertisements.Platform.Editor.EditorPlatform+<>c__DisplayClass14_0.<Show>b__0 () (at Library/PackageCache/com.unity.ads@4.0.0/Runtime/Advertisement/Platform/Editor/EditorPlatform.cs:152)
UnityEngine.Advertisements.Utilities.CoroutineExecutor.Update () (at Library/PackageCache/com.unity.ads@4.0.0/Runtime/Advertisement/Utilities/CoroutineExecutor.cs:17)

Reproducible with: 4.0.0 (2020.3.27f1, 2021.2.9f1, 2022.1.0b5, 2022.2.0a2)
Could not test with: 4.0.0 (2019.4.35f1 - Editor not responsive)

Notes:
- Not reproducible when "Enter Play Mode Options" is disabled (Editor Settings > Enter Play Mode Options)