Changed to Null ItemStack instead of null.

src/main/java/appeng/container/slot/AppEngSlot.java

@@ -85,12 +85,12 @@ public class AppEngSlot extends Slot
 	{
 		if( !this.isEnabled() )
 		{
-			return null;
+			return ItemStack.EMPTY;
 		}
 
 		if( this.inventory.getSizeInventory() <= this.getSlotIndex() )
 		{
-			return null;
+			return ItemStack.EMPTY;
 		}
 
 		if( this.isDisplay() )

src/main/java/appeng/container/slot/NullSlot.java

@@ -54,7 +54,7 @@ public class NullSlot extends Slot
 	@Override
 	public ItemStack getStack()
 	{
-		return null;
+		return ItemStack.EMPTY;
 	}
 
 	@Override
@@ -78,7 +78,7 @@ public class NullSlot extends Slot
 	@Override
 	public ItemStack decrStackSize( final int par1 )
 	{
-		return null;
+		return ItemStack.EMPTY;
 	}
 
 	@Override

src/main/java/appeng/container/slot/SlotCraftingTerm.java

@@ -200,7 +200,7 @@ public class SlotCraftingTerm extends AppEngCraftingSlot
 							return request;
 						}
 					}
-					return null;
+					return ItemStack.EMPTY;
 				}
 
 				is = r.getCraftingResult( ic );
@@ -231,7 +231,7 @@ public class SlotCraftingTerm extends AppEngCraftingSlot
 			return is;
 		}
 
-		return null;
+		return ItemStack.EMPTY;
 	}
 
 	private boolean preCraft( final EntityPlayer p, final IMEMonitor<IAEItemStack> inv, final ItemStack[] set, final ItemStack result )

src/main/java/appeng/container/slot/SlotFake.java

@@ -41,7 +41,7 @@ public class SlotFake extends AppEngSlot
 	@Override
 	public ItemStack decrStackSize( final int par1 )
 	{
-		return null;
+		return ItemStack.EMPTY;
 	}
 
 	@Override