Fix null node issue in ReactCSSTransitionGroup (facebook#6958)

(cherry picked from commit c9eb572)

src/addons/transitions/ReactCSSTransitionGroupChild.js

@@ -89,7 +89,7 @@ var ReactCSSTransitionGroupChild = React.createClass({
     CSSCore.addClass(node, className);
 
     // Need to do this to actually trigger a transition.
-    this.queueClass(activeClassName);
+    this.queueClassAndNode(activeClassName, node);
 
     // If the user specified a timeout delay.
     if (userSpecifiedDelay) {
@@ -102,26 +102,29 @@ var ReactCSSTransitionGroupChild = React.createClass({
     }
   },
 
-  queueClass: function(className) {
-    this.classNameQueue.push(className);
+  queueClassAndNode: function(className, node) {
+    this.classNameAndNodeQueue.push({
+      className: className,
+      node: node,
+    });
 
     if (!this.timeout) {
-      this.timeout = setTimeout(this.flushClassNameQueue, TICK);
+      this.timeout = setTimeout(this.flushClassNameAndNodeQueue, TICK);
     }
   },
 
-  flushClassNameQueue: function() {
+  flushClassNameAndNodeQueue: function() {
     if (this.isMounted()) {
-      this.classNameQueue.forEach(
-        CSSCore.addClass.bind(CSSCore, ReactDOM.findDOMNode(this))
-      );
+      this.classNameAndNodeQueue.forEach(function(obj) {
+        CSSCore.addClass(obj.node, obj.className);
+      });
     }
-    this.classNameQueue.length = 0;
+    this.classNameAndNodeQueue.length = 0;
     this.timeout = null;
   },
 
   componentWillMount: function() {
-    this.classNameQueue = [];
+    this.classNameAndNodeQueue = [];
     this.transitionTimeouts = [];
   },
 
@@ -132,6 +135,8 @@ var ReactCSSTransitionGroupChild = React.createClass({
     this.transitionTimeouts.forEach(function(timeout) {
       clearTimeout(timeout);
     });
+
+    this.classNameAndNodeQueue.length = 0;
   },
 
   componentWillAppear: function(done) {

src/addons/transitions/__tests__/ReactCSSTransitionGroup-test.js

@@ -291,4 +291,42 @@ describe('ReactCSSTransitionGroup', function() {
     // Testing that no exception is thrown here, as the timeout has been cleared.
     jest.runAllTimers();
   });
+
+  it('should handle unmounted elements properly', function() {
+    var Child = React.createClass({
+      render() {
+        if (!this.props.show) {
+          return null;
+        }
+        return <div />;
+      },
+    });
+
+    var Component = React.createClass({
+      getInitialState() {
+        return { showChild: true };
+      },
+
+      componentDidMount() {
+        this.setState({ showChild: false });
+      },
+
+      render() {
+        return (
+          <ReactCSSTransitionGroup
+            transitionName="yolo"
+            transitionAppear={true}
+            transitionAppearTimeout={0}
+          >
+            <Child show={this.state.showChild} />
+          </ReactCSSTransitionGroup>
+        );
+      },
+    });
+
+    ReactDOM.render(<Component/>, container);
+
+    // Testing that no exception is thrown here, as the timeout has been cleared.
+    jest.runAllTimers();
+  });
 });