fix instance null when involve async destruction

src/renderers/shared/reconciler/ReactOwner.js

@@ -95,9 +95,10 @@ var ReactOwner = {
       '`render` method, or you have multiple copies of React loaded ' +
       '(details: https://fb.me/react-refs-must-have-owner).'
     );
-    // Check that `component` is still the current ref because we do not want to
-    // detach the ref if another component stole it.
-    if (owner.getPublicInstance().refs[ref] === component.getPublicInstance()) {
+    var ownerPublicInstance = owner.getPublicInstance();
+    // Check that `component`'s owner is still alive and that `component` is still the current ref
+    // because we do not want to detach the ref if another component stole it.
+    if (ownerPublicInstance && ownerPublicInstance.refs[ref] === component.getPublicInstance()) {
       owner.detachRef(ref);
     }
   },

src/renderers/shared/reconciler/__tests__/refs-destruction-test.js

@@ -59,4 +59,52 @@ describe('refs-destruction', function() {
     ReactDOM.render(<TestComponent destroy={true} />, container);
     expect(Object.keys(testInstance.refs || {}).length).toEqual(0);
   });
+
+  it('should not error when destroying child with ref asynchronously', function() {
+    var flag = false;
+    runs(function() {
+      var Modal = React.createClass({
+        componentDidMount: function() {
+          this.div = document.createElement('div');
+          document.body.appendChild(this.div);
+          this.componentDidUpdate();
+        },
+        componentDidUpdate: function() {
+          ReactDOM.render(<div>{this.props.children}</div>, this.div);
+        },
+        componentWillUnmount: function() {
+          var self = this;
+          // some async animation
+          setImmediate(function() {
+            expect(function() {
+              ReactDOM.unmountComponentAtNode(self.div);
+            }).not.toThrow();
+            document.body.removeChild(self.div);
+            flag = true;
+          });
+        },
+        render() {
+          return null;
+        },
+      });
+      var AppModal = React.createClass({
+        render: function() {
+          return (<Modal>
+            <a ref="ref"/>
+          </Modal>);
+        },
+      });
+      var App = React.createClass({
+        render: function() {
+          return this.props.hidden ? null : <AppModal onClose={this.close}/>;
+        },
+      });
+      var container = document.createElement('div');
+      ReactDOM.render(<App />, container);
+      ReactDOM.render(<App hidden={true}/>, container);
+    });
+    waitsFor(function() {
+      return flag;
+    }, 'unmount should be ok', 10);
+  });
 });