focusedBorder property Null safety

The border to display when the InputDecorator has the focus and is not showing an error.

See also:

• InputDecorator.isFocused, which is true if the InputDecorator's child has the focus.
• InputDecoration.errorText, the error shown by the InputDecorator, if non-null.
• border, for a description of where the InputDecorator border appears.
• UnderlineInputBorder, an InputDecorator border which draws a horizontal line at the bottom of the input decorator's container.
• OutlineInputBorder, an InputDecorator border which draws a rounded rectangle around the input decorator's container.
• InputBorder.none, which doesn't draw a border.
• errorBorder, displayed when InputDecorator.isFocused is false and InputDecoration.errorText is non-null.
• focusedErrorBorder, displayed when InputDecorator.isFocused is true and InputDecoration.errorText is non-null.
• disabledBorder, displayed when InputDecoration.enabled is false and InputDecoration.errorText is null.
• enabledBorder, displayed when InputDecoration.enabled is true and InputDecoration.errorText is null.