PropertyChangedEventHandler PropertyChanged is null

Question

i am implementing PropertyChangedEventHandler PropertyChanged and it's always null. property string is right donno where is the problem here is the code i am using

public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }

public bool _playerGridVisibility ;
        public bool PlayerGridVisibility
        {
            get { return _playerGridVisibility; }
            set
            {
                _playerGridVisibility = value;
                this.OnPropertyChanged(Strings.PlayerGridVisibilityString);
            }

and in the xaml

Visibility="{Binding Path=AdsGridVisibility, Converter={StaticResource VC}}"
        }

so can anyone knows the problem ?