fillColor property Null safety
The base fill color of the decoration's container color.
When InputDecorator.isHovering is true, the hoverColor is also blended into the final fill color.
By default the fillColor is based on the current InputDecorationTheme.fillColor.
The decoration's container is the area which is filled if filled is true and bordered per the border. It's the area adjacent to icon and above the widgets that contain helperText, errorText, and counterText.
final Color? fillColor;