Fix CreativeTab ordering and erronous adding to all item groups for some items.

src/main/java/appeng/core/AEItemGroup.java

@@ -0,0 +1,40 @@
+package appeng.core;
+
+import appeng.api.AEApi;
+import appeng.api.definitions.IBlocks;
+import appeng.api.definitions.IDefinitions;
+import appeng.api.definitions.IItemDefinition;
+import net.minecraft.item.ItemGroup;
+import net.minecraft.item.ItemStack;
+import net.minecraft.util.NonNullList;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class AEItemGroup extends ItemGroup {
+
+    private final List<IItemDefinition> itemDefs = new ArrayList<>();
+
+    public AEItemGroup(String label) {
+        super(label);
+    }
+
+    @Override
+    public ItemStack createIcon() {
+        final IDefinitions definitions = AEApi.instance().definitions();
+        final IBlocks blocks = definitions.blocks();
+        return blocks.controller().stack(1);
+    }
+
+    public void add(IItemDefinition itemDef) {
+        this.itemDefs.add(itemDef);
+    }
+
+    @Override
+    public void fill(NonNullList<ItemStack> items) {
+        for (IItemDefinition itemDef : this.itemDefs) {
+            itemDef.item().fillItemGroup(this, items);
+        }
+    }
+
+}

src/main/java/appeng/core/CreativeTab.java

@@ -19,31 +19,13 @@
 package appeng.core;
 
 
-import net.minecraft.item.ItemGroup;
-import net.minecraft.item.ItemStack;
-
-import appeng.api.AEApi;
-import appeng.api.definitions.IBlocks;
-import appeng.api.definitions.IDefinitions;
-
-
 public final class CreativeTab
 {
-	public static ItemGroup instance = null;
 
-	static void init()
-	{
-		instance = new ItemGroup( "appliedenergistics2" )
-		{
+	public static AEItemGroup INSTANCE;
 
-			@Override
-			public ItemStack createIcon()
-			{
-				final IDefinitions definitions = AEApi.instance().definitions();
-				final IBlocks blocks = definitions.blocks();
-				return blocks.controller().stack(1);
-			}
-
-		};
+	public static void init() {
+		INSTANCE = new AEItemGroup( "appliedenergistics2" );
 	}
+
 }

src/main/java/appeng/core/api/definitions/ApiParts.java

@@ -220,7 +220,7 @@ public final class ApiParts implements IParts
 
 	private <T extends IPart> IItemDefinition createPart(FeatureFactory registry, String id, PartType type, Function<ItemStack, T> factory) {
 		return registry.item(id, props -> new ItemPart<>(props, type, factory))
-				.itemGroup(CreativeTab.instance)
+				.itemGroup(CreativeTab.INSTANCE)
 				.rendering(new ItemPartRendering())
 				.build();
 	}
@@ -234,7 +234,7 @@ public final class ApiParts implements IParts
 			String id = color.registryPrefix + '_' + idSuffix;
 
 			IItemDefinition itemDef = registry.item(id, props -> new ColoredPartItem<>(props, type, factory, color))
-					.itemGroup(CreativeTab.instance)
+					.itemGroup(CreativeTab.INSTANCE)
 					.rendering(new ItemPartRendering(color))
 					.build();