merge method
Implementation
ButtonThemeData merge(ButtonThemeData? style) {
if (style == null) return this;
return ButtonThemeData(
outlinedButtonStyle: style.outlinedButtonStyle ?? outlinedButtonStyle,
filledButtonStyle: style.filledButtonStyle ?? filledButtonStyle,
textButtonStyle: style.textButtonStyle ?? textButtonStyle,
defaultButtonStyle: style.defaultButtonStyle ?? defaultButtonStyle,
iconButtonStyle: style.iconButtonStyle ?? iconButtonStyle,
);
}