Guard against focusing a destroyed pane

spec/pane-spec.coffee

@@ -751,6 +751,12 @@ describe "Pane", ->
         pane2.destroy()
         expect(container.root).toBe pane1
 
+  describe "::focus()", ->
+    it "doesn't blow up if the pane has been destroyed", ->
+      pane = new Pane(paneParams())
+      pane.destroy()
+      pane.focus()
+
   describe "serialization", ->
     pane = null
 

src/pane.coffee

@@ -248,7 +248,7 @@ class Pane extends Model
   # Called by the view layer to indicate that the pane has gained focus.
   focus: ->
     @focused = true
-    @activate() unless @isActive()
+    @activate() unless @isActive() or @isDestroyed()
 
   # Called by the view layer to indicate that the pane has lost focus.
   blur: ->