According to this:
http://parmanoir.com/Taming_JavascriptCore_within_and_without_WebView
Returning NULL instead of an actual JSValueRef for a return value of a function could lead to crashes. I think I have seen related weirdness in the past when I failed to return a proper type to JSCore which resulted in very hard to understand behavior.
So this patch changes those return NULLs to return JSValueMakeUndefined().
I thought about JSObjectMakeError, but I don't fully understand the intent of the Error object and can't find any relevant real world examples of it being used. However, everybody seems to be using JSValueMakeUndefined().
This patch should be low impact since this is only triggered on an error condition.
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