iOS插件开发: (PDRCoreApp*)app.mainFrame始终为空

插件.m文件
@property (nonatomic, strong) UIView rootView;
@property (nonatomic, strong) UIViewController rootViewController;

• (PGPlugin) initWithWebView:(PDRCoreAppFrame)theWebView withAppContxt:(PDRCoreApp)app
  {
  _rootView = theWebView;//不为空
  _rootViewController = (UIViewController )app.mainFrame;//为空

  if ( self = [super initWithWebView:theWebView withAppContxt:app]) {
  [self addWebviewMode:theWebView andURL:theWebView.frameName];
  };
  return self;
  }