https://docs.flutter.io/flutter/widgets/FutureBuilder-class.html
The future must have been obtained earlier, e.g. during State.initState, State.didUpdateConfig, or State.didChangeDependencies. It must not be created during the State.build or StatelessWidget.build method call when constructing the FutureBuilder. If the future is created at the same time as the FutureBuilder, then every time the FutureBuilder's parent is rebuilt, the asynchronous task will be restarted.
A general guideline is to assume that every build method could get called every frame, and to treat omitted calls as an optimization.
(taken from https://github.com/flutter/flutter/issues/27847#issuecomment-462868299)
