fix: neve meta sidebar error on widgets.php [#4069]

[cristian-ungureanu]

Summary

The script where the error occurs shouldn't even be loaded there. It seems like "global $post_type" returns "page" on "widgets.php" so it loads the script. I did a separate check for "widgets.php" page and prevent the script from loading.
I also fixed the script so even if it loads there, it won't throw an error.

Will affect visual aspect of the product

NO

Test instructions

• Go to widgets.php / customize.php and make sure the error doesn't appear anymore.
• Go to a post type ( cpt, page, posts etc. ) single post editor and make sure the neve sidebar is still visible

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes #4069.

[pirate-bot]

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[HardeepAsrani]

Looks good, just will be to confirm the same on Customizer

[cristian-ungureanu]

Thanks, @HardeepAsrani! You were right, it was loading on customize.php too! Nice catch!

[irinelenache]

@cristian-ungureanu The reported error is gone, thank you 🚀 The notice is still visible and one of the two console errors is still there https://vertis.d.pr/i/cizSvg

[cristian-ungureanu]

Thank you, @irinelenache. It should be fine now. Sorry for this one 🙈

[irinelenache]

@cristian-ungureanu Everything's fine now, thank you 🙏

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🎉 This PR is included in version 3.6.7 🎉

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