Can't load Javascript into WKWebView reason: 'data parameter is nil'

Question

I've objective-C codes like this:

#import "QHArticle.h"
#import <WebKit/WebKit.h>

-(instancetype)init
{
    self = [super init];
    if (self)
    {
        _helperWebView = [[WKWebView alloc] init];
        NSString *path = [[NSBundle mainBundle] pathForResource:@"Article" ofType:@"js"];
        content = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];
        WKUserScript *wkUScript = [[WKUserScript alloc] initWithSource:content injectionTime:WKUserScriptInjectionTimeAtDocumentEnd forMainFrameOnly:YES];
        [_helperWebView.configuration.userContentController addUserScript:wkUScript];
        [_helperWebView reload];
    }
    return self;
}


- (NSString *)callJSMethod:(NSString *)method
{
    __block NSString *resultString = nil;
    __block BOOL finished = NO;
    [_helperWebView evaluateJavaScript:method completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                resultString = [NSString stringWithFormat:@"%@", result];
            }
        } else {
            NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
        }
        finished = YES;
    }];

    while (!finished)
    {
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }
    return resultString;
}

But i got blank screen and error: 2021-01-18 16:56:57.117808+0700 Q-UII[5400:419080] evaluateJavaScript error : A JavaScript exception occurred. Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'data parameter is nil'

Please help.