have you tried clearing twig/assets cache

Not only does "Edit Themes" fail, but themes are NOT being applied to surveys. I have 47 active surveys. All are now defaulting to whatever fallback theme applies. I have no ability to edit or control the layout.

Yes, I cleared the cache. I've tried different browsers. Same result.

just to avoid miscommunication, I'm not referring to the browser's cache. since the error mentioned the twig template engine, I thought it might be related the application's cache

limesurvey/tmp/assets (can clear this in Configuration -> Global -> General -> Clear assets cache -> Clear now)

delete these folders:
limesurvey/tmp/runtime/cache
limesurvey/tmp/runtime/HTML
limesurvey/tmp/runtime/twig_cache
limesurvey/tmp/runtime/URI

then refresh the app

can you use the theme editor on one of the base themes?

I've tried all of the above, several times. Clear now, then deleted the runtime folders as listed. Still:
500: Internal Server Error Argument 1 passed to Twig\Environment::createTemplate() must be of the type string, null given, called in /home/hgctoo5/public_html/truewiring.com/app/s/application/core/LSETwigViewRenderer.php on line 518

is the error only on your customized theme? or also on the included base themes?

I can't see any themes. When I click "Edit Themes" I immediately get the 500 error

can you try the following:

make a backup of your upload directory to your local machine (just in case)
rename your upload directory to something else
extract the attached upload.zip and copy to your app directory/upload (just the blank upload directoy from the last 6.x release)
make writeable by webserver

can you see theme editor now?

after...
delete new upload folder
move renamed old upload folder to upload

thank you for trying this troubleshooting, I cannot reproduce your issue on my installation, so I'm trying to narrow down what might be different

also if you set debug=2, is there any more information, can you screen shot?

YES! That allows me to see Themes!

Where do I set debug=2?

okay, so one of your themes has a problem... if there is no confidentiality issue, can you zip and upload you themes here and I can take a look?

debug=2 is in the config.php file

No issues with privacy. Adding themes zip here (2.8MB)

thanks! i will check with this tomorrow

set debug=>2 in application/config/config.php

// LimeSurvey developers: Set this to 2 to additionally display STRICT PHP error messages and get full access to standard templates
'debug'=>2,
'debugsql'=>0, // Set this to 1 to enanble sql logging, only active when debug = 2

To narrow the issue, I added the themes one folder at a time.

I was able to create the error when I added the hgc_default_no_resume folder. That is fantastic. Now we know where the error is coming from. NOTE: This theme is not being used in any active surveys. It can be deleted.

But it appears that all of my customization is not compatible with the new version of LS. My main theme, as you might guess, is TrueWiring_Main.

Of the 47 active surveys I have, 39 use TrueWiring_Main, 2=ECO, 1=inherit, 2=Next Initiative (extends TrueWiring_Main), 2=TrueWiring_CJCC (extends TrueWiring_Main), 1=Waypoint_Extends_TrueWiring_Main. Essentially, all of my themes (except ECO).

45 of the 47 active surveys depend on TrueWiring_Main (extends bootswatch)
the other 2 use ECO (extends bootswatch)

Please let me know if there is a way to edit my TrueWiring_Main theme to make it compatible with the new version of LS.

Please advise if there is any way I can use TrueWiring_Main and ECO themes in the current version OR if I need to rebuild the themes from scratch.

i will take a look when I have a chance

on the TrueWiring_Main, I added the following to confix.xml (after metadata section)

<compatibility>
<version>6.0</version>
</compatibility>

and it worked for me, does that work for you too?

okay, that worked on ECO also

I'm going to go ahead and close this as it's not a bug, but a problem with your custom theme. in the future, you can get some hints on how your custom theme should be updated by comparing with the updated included theme you inherit from. not ideal, but it works for me.