_completionSource getting null once I cancel to upload any document on android

[ShahriatHossain]

I am getting null when cancelling to upload pdf in this snippet of code

var tcs = Interlocked.Exchange(ref _completionSource, null);

here tcs getting null as a result app is crashing because of this code below:

tcs.SetResult(null);

[jfversluis]

Noticed this happening sometimes, tried to fix it in new code