Fix TransitionGroup error on quick toggle of components

Author: scyrizales

src/TransitionGroup.js

@@ -101,6 +101,9 @@ class TransitionGroup extends React.Component {
 
   _handleDoneAppearing = (key) => {
     let component = this.childRefs[key];
+    if (!component) {
+      return;
+    }
     if (component.componentDidAppear) {
       component.componentDidAppear();
     }
@@ -131,6 +134,9 @@ class TransitionGroup extends React.Component {
 
   _handleDoneEntering = (key) => {
     let component = this.childRefs[key];
+    if (!component) {
+      return;
+    }
     if (component.componentDidEnter) {
       component.componentDidEnter();
     }
@@ -161,7 +167,10 @@ class TransitionGroup extends React.Component {
 
   _handleDoneLeaving = (key) => {
     let component = this.childRefs[key];
-
+    
+    if (!component) {
+      return;
+    }
     if (component.componentDidLeave) {
       component.componentDidLeave();
     }