fix(object-freezing): Null is an object, but not a freezable one

Resolves T1141

spec-inbox/utils-spec.coffee

@@ -1,5 +1,7 @@
 _ = require('underscore-plus')
 Utils = require '../src/flux/models/utils'
+Thread = require '../src/flux/models/thread'
+Contact = require '../src/flux/models/contact'
 
 class Foo
   constructor: (@instanceVar) ->
@@ -13,6 +15,20 @@ class Bar extends Foo
     @moreStuff = stuff
     @method(stuff)
 
+describe "modelFreeze", ->
+  it "should freeze the object", ->
+    o =
+      a: 1
+      b: 2
+    Utils.modelFreeze(o)
+    expect(Object.isFrozen(o)).toBe(true)
+  
+  it "should not throw an exception when nulls appear in strange places", ->
+    t = new Thread(participants: [new Contact(email: 'ben@nylas.com'), null], subject: '123')
+    Utils.modelFreeze(t)
+    expect(Object.isFrozen(t)).toBe(true)
+    expect(Object.isFrozen(t.participants[0])).toBe(true)
+
 describe "deepClone", ->
   beforeEach ->
     @v1 = [1,2,3]

src/flux/models/utils.coffee

@@ -114,8 +114,9 @@ Utils =
   modelFreeze: (o) ->
     Object.freeze(o)
     for key, prop of o
-      if !o.hasOwnProperty(key) || typeof prop isnt 'object' || Object.isFrozen(prop)
-        continue
+      continue unless o.hasOwnProperty(key)
+      continue unless typeof prop is 'object' and prop isnt null
+      continue if Object.isFrozen(prop)
       Utils.modelFreeze(prop)
 
   modelReviver: (k, v) ->