MCStacker/tips/en/settings.loadItemUI.txt

Large commands containing many items can take a long time to load into MCStacker. To speed up importing, the item UI does not load and the NBT for that item is shown instead. One of the disadvantages of this approach is that the NBT sometimes contains single quotes where there would normally be a double quotes. Fortuantely this does not seem to affect the running of the generated commands in game. But, if this does cause problems, you can force the UI to load for each item so that the NBT is regenerated in its propper format. The downside will be the slow speed of importing. Another situation where you might need to make the item user interface load is when NBT syntax may have changed between versions of Minecraft. MCStacker can convert some of the changed NBT, but it needs to load the user interface first.