Temporarily fix EmptyComponents

This a workaround for the problem described in #2770. It should be
temporary because this is really just working around the real problem.

src/core/ReactEmptyComponent.js

@@ -30,10 +30,21 @@ var ReactEmptyComponentInjection = {
 var ReactEmptyComponentType = function() {};
 ReactEmptyComponentType.prototype.componentDidMount = function() {
   var internalInstance = ReactInstanceMap.get(this);
+  // TODO: Make sure we run these methods in the correct order, we shouldn't
+  // need this check. We're going to assume if we're here it means we ran
+  // componentWillUnmount already so there is no internal instance (it gets
+  // removed as part of the unmounting process).
+  if (!internalInstance) {
+    return;
+  }
   registerNullComponentID(internalInstance._rootNodeID);
 };
 ReactEmptyComponentType.prototype.componentWillUnmount = function() {
   var internalInstance = ReactInstanceMap.get(this);
+  // TODO: Get rid of this check. See TODO in componentDidMount.
+  if (!internalInstance) {
+    return;
+  }
   deregisterNullComponentID(internalInstance._rootNodeID);
 };
 ReactEmptyComponentType.prototype.render = function() {

src/core/__tests__/ReactEmptyComponent-test.js

@@ -231,4 +231,38 @@ describe('ReactEmptyComponent', function() {
       'Invariant Violation: React.render(): Invalid component element.'
     );
   });
+
+  it('does not break when updating during mount', function() {
+    var Child = React.createClass({
+      componentDidMount() {
+        this.props.onMount && this.props.onMount();
+      },
+      render() {
+        if (!this.props.visible) {
+          return null;
+        }
+
+        return <div>hello world</div>;
+      }
+    });
+
+    var Parent = React.createClass({
+      update() {
+        this.forceUpdate();
+      },
+      render() {
+        return (
+          <div>
+            <Child key="1" visible={false} />
+            <Child key="0" visible={true} onMount={this.update} />
+            <Child key="2" visible={false} />
+          </div>
+        );
+      }
+    });
+
+    expect(function() {
+      ReactTestUtils.renderIntoDocument(<Parent/>)
+    }).not.toThrow();
+  });
 });