App crash by using async task download image on Mobile[IOS][UE4.10]

Hi, I am using async take download image NODE in blueprint. So, we could change image from URL by press button on Iphone. But issue is, after change image several times, the app crash back to main screen.

To me, seem download too much image, so the memory cache is out of range maybe?

Please help me fix this. Give some direction. Many thanks.