fix a color setting issue

@restore_textcursor

def set_textblk_color(blkitem: TextBlkItem, cursor: QTextCursor, rgb: List):

    if not blkitem.document().isEmpty():

        fraghtml = cursor.selection().toHtml()

        cursor.insertHtml(set_html_color(fraghtml, rgb))

    else:

        fmt = cursor.charFormat()

        fmt.setForeground(QColor(*rgb))

        cursor.setCharFormat(fmt)

@restore_textcursor