Cancel upload request continuous (Dropbox api V2) in iOS

Hi Team DB,

I have issue about cancel Upload request. Please help me :)

Spec: I have a feature upload 1 file to dropbox, when click on button Upload to upload I display 1 loading view.

In loading view, i have a button Cancel (use for cancel uploading). But when I press continuous Upload then press Cancel then press Upload and then press Cancel again. I do it about 4-5 times.

==> App crash

It seem relate to dropbox api. Please see attachment below 

https://www.dropbox.com/s/3ebv35r3kj4t0aw/Screen%20Shot%202016-11-15%20at%202.35.58%20PM.png?dl=0

Thanks,

Vang Doan