iOS crash on first RewaerdVideo Ad (AdMob)

Today I tried to export my game to iOS (I use the latest version of Unity 2019.2.8) after having already exported it and tested it on Android.
I use the latest version of AdMob for Unity (version 4.0.0), to show some advertisements within my game.
After compiling the game on xCode, the game starts correctly, but when I go to see a Rewarded Ads for the first time, the game is created immediately.
Later if I try to restart the game, the Rewarded Ads seems to display correctly, without having to crash anymore. As for the display of normal advertising banners, there is no problem.
These are the error screens I get on xCode when the game crashes:



I want to clarify that on Android I tried several times, and I don’t have any app crashes. This occurs only if I export to iOS (tested with the iPhone 6s and 11 Pro).
Can anyone help me? Unfortunately I need to complete this project as soon as possible, but this problem does not allow me to publish it!:frowning:

Is there no one who has had the same problem, or can he help me?
These are days I’ve been trying to solve to publish my game, but I can’t find a solution …:frowning: