Only move legit items to the top of the stack

Fixes #11002 cc @natalieogle

spec/pane-spec.coffee

@@ -337,6 +337,10 @@ describe "Pane", ->
       expect(pane.itemStack).toEqual [item2]
       expect(pane.getActiveItem()).toBe item2
 
+      pane.destroyItem(item2)
+      expect(pane.itemStack).toEqual []
+      expect(pane.getActiveItem()).toBeUndefined()
+
     it "invokes ::onWillDestroyItem() observers before destroying the item", ->
       events = []
       pane.onWillDestroyItem (event) ->

src/pane.coffee

@@ -307,6 +307,7 @@ class Pane extends Model
 
   # Add item (or move item) to the end of the itemStack
   addItemToStack: (newItem) ->
+    return unless newItem?
     index = @itemStack.indexOf(newItem)
     @itemStack.splice(index, 1) unless index is -1
     @itemStack.push(newItem)