value property Null safety
If non-null, the value of this progress indicator.
A value of 0.0 means no progress and 1.0 means that progress is complete. The value will be clamped to be in the range 0.0-1.0.
If null, this progress indicator is indeterminate, which means the indicator displays a predetermined animation that does not indicate how much actual progress is being made.
final double? value;