[1.16.5] Clearing and adding new slots in a container

[ImNotJahan]

Like what the title says I need the slots in a container to all be removed so I can then add in new ones. I have a method for whenever they should change, and it just runs

slots.clear();

switch(menuIndex)
        {
            case 0:
                setupSwordInv(playerInventory);
                break;
    ... and more of this for a few more lines

The setupSwordInv functions and others just place the slots, and I know that each function is being ran when it's meant to but all the slots seem to be in the exact same position from what they're set at during the start. I'm guessing there's something I have to do with syncing the container and screen after changing them, but I'm not sure what so any help is appreciated. 

[ImNotJahan]

I've tried changing the x and y of the slots instead, but whenever I do this I get 

java.lang.NullPointerException: Rendering screen
	at net.minecraft.inventory.container.Slot.getItem(Slot.java:53)

whilst rendering

[Luis_ST]

did you update the client?

[ImNotJahan]

Okay yea that got it looking correctly, and the NullPointerException was just due to me forgetting to stop clearing the slots