Work around Carpet crash when remove_spawn_chunks is enabled

Related: #390

common/src/main/java/org/embeddedt/modernfix/common/mixin/perf/remove_spawn_chunks/SortedArraySetMixin.java

@@ -0,0 +1,25 @@
+package org.embeddedt.modernfix.common.mixin.perf.remove_spawn_chunks;
+
+import com.llamalad7.mixinextras.injector.wrapoperation.Operation;
+import com.llamalad7.mixinextras.injector.wrapoperation.WrapOperation;
+import net.minecraft.util.SortedArraySet;
+import org.embeddedt.modernfix.ModernFix;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+
+@Mixin(SortedArraySet.class)
+public class SortedArraySetMixin<T> {
+    /**
+     * @author embeddedt
+     * @reason Make add() not crash with a null key, since some mods (Carpet) assume there will always be a spawn ticket,
+     * and then assume the reference they have is non-null (it can be null with this option enabled).
+     */
+    @WrapOperation(method = "add", at = @At(value = "INVOKE", target = "Lnet/minecraft/util/SortedArraySet;findIndex(Ljava/lang/Object;)I"), require = 0)
+    private int checkStatus(SortedArraySet<T> instance, T object, Operation<Integer> original) {
+        if(object == null) {
+            ModernFix.LOGGER.error("Attempted to insert a null key into SortedArraySet, ignoring");
+            return 0;
+        }
+        return original.call(instance, object);
+    }
+}