Doors on covered litter boxes should be permanently removed. Cats should not be completely enclosed when they use the litter box, they need to be able to see out, to feel comfortable. Your kitten is afraid to come out because she can't see whether it is safe outside the box. Please remove the doors!

Most cat litters are quite dusty and it is not fair for the cats to have to breathe in all that dust when they are using the boxes, digging holes and burying.

A door does not help with containing litter; litter is brought out on the cats' feet. I recommend buying litter mats to put in front of the litter boxes. They have a rough surface which catches the bits of litter off their feet as they exit the box. They work perfectly in my house. This is the kind of thing:

https://www.amazon.co.uk/Vivaglory-90x60CM-Non-slip-Waterproof-PHTHALATE/dp/B07FQNCZZF/ref=sr_1_6?crid=1XDIMCLJL2MYR&dchild=1&keywords=litter+tray+mats+washable&qid=1596722470&sprefix=litter+tray+mats+,aps,149&sr=8-6

 

but the doors are transparent

 

@skill iit
I agree about using litter mats or if you cannot find the rubber type of litter mat where you are, you can try a bath mat.
I took the door off my walk in type litter box & just use a litter mat. It helps but not fully. I find pellets track less than clay or similar.

 

I have tried closed closed doors, my cat didn't like so much, than we bought a big litter tray, completely open and she's been very adapted to it. And it leaves no mess

 

I got the same litter tray with the same amount of litter....my kittens dig in so aggressively like there is no tomorrow... especially after I scoop out all of their stool and urine...

so I come and see the mat full of litter and everything....sometimes they dig their poop out