This issue is caused by the WordPress wpautop function which wraps lines with paragraph tags, and the function is being executed in the text widget since it is present in the text editor by default (this was introduced in WordPress version 4.8).
In order to solve this, you can remove all spaces within text widgets, however if this doesn’t appear to work for you for any reason, or in case you do not want to go through all the theme widgets manually, you can simply add the following line of code inside the theme folder / functions.php:
remove_filter('widget_text_content', 'wpautop');
In case you wish to modify this permanently and for all future updates, you should add that line inside the child theme folder / functions.php file as well.