TextEdit uses the same mechanism to restore windows if the checkbox is unchecked in System Preferences.

Does TextEdit correctly restore windows?

Does a reboot help?

You can also try to delete the folder that contains the saved application state:

• Close Textastic
• Delete the folder ~/Library/Containers/com.textasticapp.textastic-mac/Data/Library/Saved Application State

Hmm, weird. 

What happens when you go to System Preferences -> General and uncheck the checkbox called "Close windows when quitting an app"?

That sounds like the Working Copy file provider extension crashes. 

Please try to reboot your iPad. If that doesn't help, please contact Anders Borum, the author of Working Copy and ask him for advice.

Yes, I've seen that. That's excellent news.

You can add custom syntax definitions to Textastic by using TextMate bundles. See
http://www.textasticapp.com/v6/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html

A quick search for "taskpaper tmbundle" in Google brought up this TextMate bundle which should work:

https://github.com/jessmartin/taskpaper-tmbundle

No, currently this is not supported, sorry. Previously this wasn't allowed by Apple at all, but they recently made some changes to the App Store guidelines which I'll look into.

You can add custom syntax definitions to Textastic by using TextMate bundles. See
http://www.textasticapp.com/v6/manual/lessons/How_can_I_add_my_own_syntax_definitions__themes_and_templates.html

A quick search for "Kotlin tmbundle" in Google brought up this TextMate bundle which should work:

https://github.com/sargunster/kotlin-textmate-bundle

Ok, I just saw that on macOS, the code completion window is not visible if the window is maximized and you are editing in the last line. That's definitely a bug. I've added it to my todo list.