Error: A value of type '#lib1::TickerFuture'

[edeuss]

I keep getting this error when I try to build:
`file:///C:/Users/Elias/Documents/flutter/.pub-cache/hosted/pub.dartlang.org/native_ui-0.0.5/lib/src/ios/button.dart:185:9: Error: A value of type '#lib1::TickerFuture' can't be assigned to a variable of type 'dart.async::Futuredart.core::Null'.

Try changing the type of the left hand side, or casting the right hand side to 'dart.async::Futuredart.core::Null'.
? _animationController.animateTo(1.0, duration: kFadeOutDuration)
^
`