title: Error: Undefined symbol for architecture x86_64: "OBJC_CLASS_$_InventoryItem", referenced from: objc-class-ref in AppDelegate.o
body: <p><a href="https://i.stack.imgur.com/XMduG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XMduG.png" alt="This is the exact error."></a></p>
  
  <p>What should I do to solve this problem?  I created both the InventoryItem.h &amp; InventoryItem.c files so I don't understand the problem.</p>