Fix NPE crash when changing focus using TAB (#201)

`SearchFieldEntry.children()` was returning `List.of(editBox)`, which itself returns an "unmodifiable" list.

Unmodifiable lists do not permit null entries, and [`indexOf()` will throw a `NullPointerException`][1] if the specified element is `null` and the list does not permit `null` elements.
This reveals a bug in vanilla's `ContainerEventHandler#changeFocus(boolean)` method, where `indexOf()` is called without a null-check, causing the crash.

Instead, we can use `Collections.singletonList()` which returns a `SingletonList`. Still immutable, but no silly NPEs.

Fixes #176

[1]: https://docs.oracle.com/javase/8/docs/api/java/util/List.html#indexOf-java.lang.Object-

common/src/main/java/me/shedaniel/clothconfig2/gui/widget/SearchFieldEntry.java

@@ -132,11 +132,11 @@ public Optional<Object> getDefaultValue() {
 
     @Override
     public List<? extends NarratableEntry> narratables() {
-        return List.of(editBox);
+        return Collections.singletonList(editBox);
     }
 
     @Override
     public List<? extends GuiEventListener> children() {
-        return List.of(editBox);
+        return Collections.singletonList(editBox);
     }
 }