Fixes NPE on random button smashing

src/main/java/appeng/items/parts/ItemMultiPart.java

@@ -301,6 +301,8 @@ public class ItemMultiPart extends AEBaseItem implements IPartItem, IItemGroup
 			if( stack.getItem() == this )
 			{
 				PartType pt = this.getTypeByStack( stack );
+				if ( pt != null )
+				{
 					switch( pt )
 					{
 						case ImportBus:
@@ -317,6 +319,7 @@ public class ItemMultiPart extends AEBaseItem implements IPartItem, IItemGroup
 					}
 				}
 			}
+		}
 
 		if( group && importBus && exportBus )
 			return GuiText.IOBuses.getUnlocalized();