only invoke didChange if a node is still present.

packages/ember-metal/lib/chains.js

@@ -287,9 +287,13 @@ ChainNode.prototype = {
 
   willChange(events) {
     var chains = this._chains;
+    var node;
     if (chains) {
       for (var key in chains) {
-        chains[key].willChange(events);
+        node = chains[key];
+        if (node !== undefined) {
+          node.willChange(events);
+        }
       }
     }
 
@@ -389,7 +393,10 @@ export function finishChains(obj) {
         chainNodes = chainWatchers[key];
         if (chainNodes) {
           for (var i = 0, l = chainNodes.length; i < l; i++) {
-            chainNodes[i].didChange(null);
+            var node = chainNodes[i];
+            if (node) {
+              node.didChange(null);
+            }
           }
         }
       }

packages/ember-metal/tests/chains_test.js

@@ -1,6 +1,9 @@
 import { addObserver } from 'ember-metal/observer';
+import { get } from 'ember-metal/property_get';
 import { finishChains } from 'ember-metal/chains';
-
+import { defineProperty } from 'ember-metal/properties';
+import computed from 'ember-metal/computed';
+import { propertyDidChange } from 'ember-metal/property_events';
 QUnit.module('Chains');
 
 QUnit.test('finishChains should properly copy chains from prototypes to instances', function() {
@@ -14,3 +17,52 @@ QUnit.test('finishChains should properly copy chains from prototypes to instance
 
   ok(obj['__ember_meta__'].chains !== childObj['__ember_meta__'].chains, 'The chains object is copied');
 });
+
+
+QUnit.test('observer and CP chains', function() {
+  var obj = { };
+
+  defineProperty(obj, 'foo', computed('qux.[]', function() { }));
+  defineProperty(obj, 'qux', computed(function() { }));
+
+  // create DK chains
+  get(obj, 'foo');
+
+  // create observer chain
+  addObserver(obj, 'qux.length', function() { });
+
+  /*
+             +-----+
+             | qux |   root CP
+             +-----+
+                ^
+         +------+-----+
+         |            |
+     +--------+    +----+
+     | length |    | [] |  chainWatchers
+     +--------+    +----+
+      observer       CP(foo, 'qux.[]')
+  */
+
+
+  // invalidate qux
+  propertyDidChange(obj, 'qux');
+
+  // CP chain is blown away
+
+  /*
+             +-----+
+             | qux |   root CP
+             +-----+
+                ^
+         +------+xxxxxx
+         |            x
+     +--------+    xxxxxx
+     | length |    x [] x  chainWatchers
+     +--------+    xxxxxx
+      observer       CP(foo, 'qux.[]')
+  */
+
+  get(obj, 'qux'); // CP chain re-recreated
+  ok(true, 'no crash');
+});