Issue is mentioned here at the bottom of the page under “WebKit Known Issues” : https://developer.apple.com/library/prerelease/ios/releasenotes/General/RN-iOSSDK-8.0/index.html
This issue is also reported on Apache Cordova site at :
Some alternative solutions to the issue are listed here :
So Cordova/PhoneGap App cannot be tested on the iOS8.0 beta before a stable release of the software.
18/06/2014 Update : Xcode beta2 released and it also have the same issue.
08/07/2014 Update : Xcode beta3 released and issue is resolved now.