I'll explain the title here. So when the script is invoked with the standard dmenu_run script, everything works fine but with my dmenu script which should basically work the same way as the former, I get this error from jq:
jq: error (at <stdin>:14): can't iterate over null (null)
How I found out about this bug is that if I run your two scripts in the terminal, it works as expected but when I put a key binding in the header file for dwm, I was getting empty prompts i.e. the search results weren't being displayed. And apparently that was happening because of the fq error.
The only workaround I have right now is to invoke the standard dmenu_run script, type your script's name and do the search from there.
Since this is a new project, the issues are expected. However, your script is very helpful. Thanks.
Also, please check this video.
All the requirements are fulfilled.

Since using Youtoube API for simple search seems like too much trouble (and is limited in the number of uses anyways), why not use a webpage scraper?

By a quick google, I found youtube-search (pip3 install youtube-search), and I changed the bash script to use the following python script instead of querying youtube API:

#!/usr/bin/env python3
from youtube_search import YoutubeSearch
from sys import argv

search_terms = str(argv)

results=YoutubeSearch(search_terms, max_results=10).to_dict()
urlString = '\n'.join((r['title']+' '+r['id'] for r in results))
print(urlString)

main bash lines then become:

urlstring=$(./new_python_script_name.py "${query}")
mpv "https://youtu.be/$( echo "${urlstring}" \
    | fzf --with-nth='1..-2' +m \
    | awk '{print $NF}' \
)"

It works great for me. It does require some polishing regarding the location of both scripts which influences how you call python from bash etc. The two scripts could probably be even merged into one (heredocs in bash or use python to call fzf and mpv) for extra points :)

So I'm just leaving this idea here with you, if I ever foolproof it, I'l turn it into a merge request.

Hi Bugswriter, thanks for the script!

When running ./dmenyt or ./myyt it prompts for the search query. Regardless of what I enter I receive the syntax error

$ ./myyt
Search query: test
./myyt: 9: Syntax error: redirection unexpected

Dependencies:

fzf is already the newest version (0.20.0-1).
mpv is already the newest version (0.32.0-1ubuntu1).
suckless-tools is already the newest version (44-1). [This contains dmenu]
youtube-dl is already the newest version (2020.03.24-1).

API key is properly inserted as variable in terminal,
output of echo:

user@desktop$: echo $YT_API_KEY
AIzaSyB-rGfbeC2w8dMxxxxxxx