iOS game crashes instead of showing an ad after Unity Ads package was updated to the latest version (3.7.1 as of writing)

Basically, it says in log output in Xcode: Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[NSString stringWithUTF8String:]: NULL cString' terminating with uncaught exception of type NSException I have no idea what to do here. Anyone had similar situation? I don't know how to fix it. Everything works well with older Unity Ads version, it shows ads and all.