Fix crash caused by pressing TAB

[MattSturgeon]

Fixes #176, which was caused by SearchFieldEntry.children() using List.of() to form the list of children.

List.of() returns an unmodifiable list, which doesn't permit null members. This means that List.indexOf() will throw a NullPointerException when null is passes to it, reveals a bug in the vanilla method ContainerEventHandler#changeFocus(boolean).

ContainerEventHandler.changeFocus() with comments regarding the vanilla bug

// decompiled using loom & fernflower, manually cleaned & commented
boolean changeFocusFernFlower(boolean moveForward) {
    GuiEventListener currentlyFocused = this.getFocused();
    boolean focusedIsNotNull = currentlyFocused != null;
    
    // First try letting the currently focused gui handle changing focus
    if (focusedIsNotNull && currentlyFocused.changeFocus(moveForward)) {
        return true;
    }
    
    // Otherwise iterate over the children
    List<? extends GuiEventListener> list = this.children();
    
    // We want to start iterating from the next/previous gui to `currentlyFocused`
    // This is where the crash occurs - if `currentlyFocused` is null & `list` does not allow null entries:
    int index = list.indexOf(currentlyFocused); // NullPointerException
    int start;
    // This is a vanilla bug:
    // list.indexOf() should only be run if focusedIsNotNull
    if (focusedIsNotNull && index >= 0) {
        start = index + (moveForward ? 1 : 0);
    } else {
        start = moveForward ? 0 : list.size();
    }
    /* Vanilla fix:
    // Mojang could fix this by only calling indexOf() when `focusedIsNotNull`:
    int index, start;
    if (focusedIsNotNull && (index = list.indexOf(guiEventListener)) >= 0) {
        start = index + (moveForward ? 1 : 0);
    } else {
        start = moveForward ? 0 : list.size();
    }
     */

    // Get an iterator starting at the appropriate index
    ListIterator<? extends GuiEventListener> iterator = list.listIterator(start);

    // Get a reference to either iterator.hasNext() or iterator.hasPrevious()
    BooleanSupplier hasAnother;
    if (moveForward) {
        Objects.requireNonNull(iterator);
        hasAnother = iterator::hasNext;
    } else {
        Objects.requireNonNull(iterator);
        hasAnother = iterator::hasPrevious;
    }

    // Get a reference to either iterator.next() or iterator.previous()
    Supplier<? extends GuiEventListener> another;
    if (moveForward) {
        Objects.requireNonNull(iterator);
        another = iterator::next;
    } else {
        Objects.requireNonNull(iterator);
        another = iterator::previous;
    }

    // Iterate until a gui accepts focus, or we run out of guis
    GuiEventListener gui;
    do {
        if (!hasAnother.getAsBoolean()) {
            this.setFocused(null);
            return false;
        }
        
        gui = another.get();
    } while(!gui.changeFocus(moveForward));

    this.setFocused(gui);
    return true;
}

This PR fixes the issue by having SearchFieldEntry.children() use Collections.singletonList() instead of List.of(). Singleton lists permit null values, so indexOf() doesn't need to throw any exceptions when we ask for the index of null.

I've also included a couple commits fixing mapping names I stumbled across while debugging. I can open a separate PR for those if you like, but I figured so long as the commits don't get squashed together when merging one small PR should be ok. EDIT: moved to #202

[shedaniel]

Thanks! LGTM

[5HT2]

@shedaniel merge?

[shedaniel]

Thanks for reminding me! I will merge it this weekend!

[MattSturgeon]

@shedaniel it'd be great to get this merged 😄