-[NSURLSession dataTaskWithRequest:] throws weird error when calling shared instance of JSContext

Question

I am trying to execute JavaScript in a hooked method from the NSURLSession class (cluster) which returns a LocalDataTask. For some reason my iOS tweak causes apps to crash when they call the method that I've hooked (-[NSURLSession dataTaskWithRequest:]) using Theos.

I have no idea why this is happening because I am not getting any useful error messages.

The reason why I am grabbing the JSContext from a UIWebView is because I want to make use of all the JavaScript objects from WebKit (e.g. XMLHttpRequest).

I have the following code in my Tweak.xm file:

#import <WebKit/WebKit.h>
#import <JavaScriptCore/JavaScriptCore.h>

@interface myJSContext : NSObject
+(JSContext *)sharedContext;
@end

@implementation myJSContext

+(JSContext *)sharedContext {
  static JSContext * jsCtxt = nil;
  static UIWebView * webView = nil;
  static dispatch_once_t onceToken;
  if (!webView) {
    dispatch_once(&onceToken, ^{
      webView = [[%c(UIWebView) alloc] init];
      jsCtxt = [webView valueForKeyPath:@"documentView.webView.mainFrame.javaScriptContext"];
    });
  }
  return jsCtxt;
}

@end

%hook SpringBoard

-(void)applicationDidFinishLaunching:(id)arg1 {
  %orig;
  NSLog(@"%@", [myJSContext sharedContext]); // prints the pointer as expected
  NSLog(@"%@", [[myJSContext sharedContext] evaluateScript:@"'js test result'.toString()"]); // successfully executes JavaScript as expected
}

%end

%hook NSURLSession

-(id)dataTaskWithRequest:(id)arg1 {
  NSLog(@"%@", [myJSContext sharedContext]); // this causes an error, I've tried calling this from the main and global thread which didn't work either
  return %orig;
}

%end

I don't see any useful error messages, but I will include the logs of potential relevance.

EDIT: I cannot post all the log messages here because this site thinks it "looks like spam"...

Proof that the JSContext instance is created and functions:

Aug 17 07:04:55 SpringBoard(TestTweak.dylib)[7402] <Notice>: js test result

The only error messages that I can see are the following:

Aug 17 07:07:00 MobileSafari(SafariServices)[7464] <Error>: Displaying webpage loading error to user: Error Domain=WebKitErrorDomain Code=300, networkTaskDescription: (null).

Aug 17 07:07:00 MobileSafari(WebKit)[7464] <Error>: 0x104fcbc00 - ProcessAssertion::processAssertionWasInvalidated()

Aug 17 07:16:41 runningboardd(RunningBoard)[39] <Notice>: [xpcservice<com.apple.WebKit.Networking>:7504] Error 45 setting darwin role to NonUserInteractive: Operation not supported, falling back to setting priority

Aug 17 07:16:41 com.apple.WebKit.Networking(CFNetwork)[7504] <Notice>: Faulting in NSHTTPCookieStorage singleton

Aug 17 07:16:41 com.apple.WebKit.Networking(CFNetwork)[7504] <Notice>: Faulting in CFHTTPCookieStorage singleton

None of those error messages explicitly tell me what went wrong.

Why does this work fine when called from a hooked method that belongs to SpringBoard, but not when called from a hooked method that belongs to NSURLSession?