Changed to Null ItemStack instead of null.

src/main/java/appeng/core/features/ColoredItemDefinition.java

@@ -70,7 +70,7 @@ public final class ColoredItemDefinition implements AEColoredItemDefinition
 
 		if( is == null )
 		{
-			return null;
+			return ItemStack.EMPTY;
 		}
 
 		return is.stack( stackSize );

src/main/java/appeng/core/features/registries/P2PTunnelRegistry.java

@@ -162,7 +162,7 @@ public final class P2PTunnelRegistry implements IP2PTunnelRegistry
 
 		if( item == null )
 		{
-			return null;
+			return ItemStack.EMPTY;
 		}
 
 		final ItemStack myItemStack = new ItemStack( item, 1, meta );

src/main/java/appeng/core/features/registries/grinder/GrinderRecipeManager.java

@@ -233,7 +233,7 @@ public final class GrinderRecipeManager implements IGrinderRegistry, IOreListene
 		{
 			return is.copy();
 		}
-		return null;
+		return ItemStack.EMPTY;
 	}
 
 	private int getDustToOreRatio( final String name )