feat:avoid iOS dealloc JSValue in main thread,this may cause crash in JavaScriptCore

doric-iOS/Example/Example/ViewController.m

@@ -23,6 +23,7 @@ @implementation ViewController
 - (void)viewDidLoad {
     [super viewDidLoad];
     self.title = @"Doric Demo";
+    [self.navigationController.navigationBar setBackgroundImage:UIImageWithColor(UIColor.whiteColor) forBarMetrics:UIBarMetricsDefault];
     NSString *path = [[NSBundle mainBundle] bundlePath];
     NSString *demoPath = [path stringByAppendingPathComponent:@"src"];
     NSFileManager *mgr = [NSFileManager defaultManager];