Fix crash when somehow we end up on a NFTPage with no slots

[modersohn]

• happens, when URL nftPageNumber is edited to go beyond end
• make accountNFTSlots non-nullable and handle cases to avoid they become null
• minor change remove unnecessary ! when calling GetAccountTotalNfts

[modersohn]

@danielsolistensvik would you give the changes a quick look?

[daniel-soli]

Looks good to avoid these cases 👍🏻