cleanCache时崩溃

NSSet *websiteDataTypes = [WKWebsiteDataStore allWebsiteDataTypes];
NSDate *dateFrom = [NSDate dateWithTimeIntervalSince1970:0];
 [[WKWebsiteDataStore defaultDataStore] removeDataOfTypes:websiteDataTypes modifiedSince:dateFrom completionHandler:nil];

websiteDataTypes为nil时崩溃