a TreeView without a root path can be serialized

spec/extensions/tree-view-spec.coffee

@@ -58,6 +58,9 @@ describe "TreeView", ->
       it "does not create a root node", ->
         expect(treeView.root).not.toExist()
 
+      it "serializes without throwing an exception", ->
+        expect(-> treeView.serialize()).not.toThrow()
+
       it "creates a root view when the project path is created", ->
         rootView.open(require.resolve('fixtures/sample.js'))
         expect(treeView.root.getPath()).toBe require.resolve('fixtures')

src/extensions/tree-view/tree-view.coffee

@@ -60,7 +60,7 @@ class TreeView extends View
     @focus() if @focusAfterAttach
 
   serialize: ->
-    directoryExpansionStates: @root.serializeEntryExpansionStates()
+    directoryExpansionStates: @root?.serializeEntryExpansionStates()
     selectedPath: @selectedEntry()?.getPath()
     hasFocus: @is(':focus')
     attached: @hasParent()