Index out of bounds when scrolling to the very end

draggable_scrollbar 0.0.4

Flutter version 1.0.0
Framework revision 5391447fae (3 weeks ago), 2018-11-29 19:41:26 -0800
Engine revision 7375a0f414
Dart version 2.1.0 (build 2.1.0-dev.9.4 f9ebf21297)

When I use the code suggested in the example:

```
labelTextBuilder: (offset) {
final int currentItem = controller.hasClients
? (controller.offset /
controller.position.maxScrollExtent *
numItems)
.floor()
: 0;

return Text("$currentItem");
}
```
and scroll all the way to the end of the list, the index gets out of bounds. I was able to solve this problem by adding 1 pixel to the _controller.position.maxScrollExtent:

```
labelTextBuilder: (offset) {
final int currentItem = controller.hasClients
? (controller.offset /
(controller.position.maxScrollExtent + 1) *
numItems)
.floor()
: 0;

return Text("$currentItem");
}
```