I want to create patches and mods for FOLON but whenever I try to work with LondonWorldSpace.esm the Creation Kit crashes with an Assertion Message.

"Assertion
Out of Handle Array Entries"

Things of note:

-I downgraded the CK so it works with the older Fallout 4 Version.

-DLCCoast and other worldspaces work fine.

If you get that error, then you are trying to load too many form records to the kit, so that it cannot read all of them. It has a limitation on this. What you can do is to load the plugin without masters (so load the plugin as standalone)

see if using this plugin would help you as well; https://www.nexusmods.com/fallout4/mods/51165