message: Check for null value on change
diff: @@ -41,7 +41,7 @@ export function useKeyboardPickerState(props: BaseKeyboardPickerProps, options:
  }, [format, props, props.value, utils]);
  function handleChange(date: MaterialUiPickersDate) {
  - const dateString = utils.format(date, format);
  + const dateString = date === null ? null : utils.format(date, format);
  props.onChange(date, dateString);
  }
  