picker.SelectedAssets.Clear(); not working?

[nguyenthanhliemfc]

I want to clear selected image of last time.
I have try set _preselectedAssets = null at the end of Finished func, It's work BUT what happend when user open picker many time?

I try generate picker onetime in ctor and try clear pre-selected assets by picker.SelectedAssets.Clear(); but it's not working.