WordPress 4.8 has just been released, and many web designers have reported that their layout have been mucked up with this release.
*** UPDATE ***
Since writing this blog post I have written a plugin – https://wordpress.org/plugins/add-paragraphs-option-to-text-widget/ – that solves this problem in a very neat way.
There could be multiple reasons for this, but one reason is that text widgets now always automatically add paragraphs to the html.
In theory, every thing should be OK if you don’t edit the text widget, but as soon as you do, any blank lines will get <p> inserted, and if for instance you are using the text widget for a <script> and there are blank lines the insertion of <p> will break the script.
Web developers have relied on the text widget to be able to add raw code, but no longer.
However there is a quick fix, that will stop paragraphs being insert, but it applies across all text widgets.
remove_filter( 'widget_text_content', 'wpautop' );
Add this to an appropriate place in your ( child ) theme’s functions.php
Of course, if this is a bit beyond you, you can contact me and I can quote to help support your WordPress site.