Returns the Unicode character (grapheme cluster) completed by this keystroke, if any.
This will only return a character if this keystroke, combined with any preceding keystroke(s), generated a character, and only on a "key down" event. It will return null if no character has been generated by the keystroke (e.g. a "dead" or "combining" key), or if the corresponding key is a key without a visual representation, such as a modifier key or a control key.
This can return multiple Unicode code points, since some characters (more accurately referred to as grapheme clusters) are made up of more than one code point.
character doesn't take into account edits by an input method editor
(IME), or manage the visibility of the soft keyboard on touch devices. For
composing text, use the TextField or CupertinoTextField widgets, since
those automatically handle many of the complexities of managing keyboard
final String character