fix: document head null error

[leafOfTree]

This is to fix https://github.com/sveltejs/svelte-devtools/issues/232

Add check for document.head. If it's null, call appendChild on the event DOMContentLoaded

[ignatiusmb]

Thanks for the PR! I've added some context in https://github.com/sveltejs/svelte-devtools/issues/232#issuecomment-2370838203 but it seems like we'll need to find another fix as this would effectively invalidate the injectImmediately setting. Perhaps we could try removing the setting altogether, but I'll need to check what behaviors was prevented by enabling it in the first place

[ignatiusmb]

Thanks for this, #238 has been merged and should fix the issue