Fix error thrown with Preact@8

Occurs when `CSSTransitionGroup` is provided with `children` containing `null`

Author: dharFr

src/CSSTransitionGroup.js

@@ -12,7 +12,7 @@
 
 
 import { h, cloneElement, Component } from 'preact';
-import { getKey } from './util';
+import { getKey, filterNullChildren } from './util';
 import { mergeChildMappings, isShownInChildren, isShownInChildrenByKey, inChildren, inChildrenByKey } from './TransitionChildMapping';
 import { CSSTransitionGroupChild } from './CSSTransitionGroupChild';
 
@@ -42,10 +42,10 @@ export class CSSTransitionGroup extends Component {
 	}
 
 	componentWillReceiveProps({ children, exclusive, showProp }) {
-		let nextChildMapping = (children || []).slice();
+		let nextChildMapping = filterNullChildren(children || []).slice();
 
 		// last props children if exclusive
-		let prevChildMapping = exclusive ? this.props.children : this.state.children;
+		let prevChildMapping = filterNullChildren(exclusive ? this.props.children : this.state.children);
 
 		let newChildren = mergeChildMappings(
 			prevChildMapping,
@@ -118,7 +118,7 @@ export class CSSTransitionGroup extends Component {
 
 	_handleDoneEntering(key) {
 		delete this.currentlyTransitioningKeys[key];
-		let currentChildMapping = this.props.children,
+		let currentChildMapping = filterNullChildren(this.props.children),
 			showProp = this.props.showProp;
 		if (!currentChildMapping || (
 			!showProp && !inChildrenByKey(currentChildMapping, key)
@@ -157,7 +157,7 @@ export class CSSTransitionGroup extends Component {
 	_handleDoneLeaving(key) {
 		delete this.currentlyTransitioningKeys[key];
 		let showProp = this.props.showProp,
-			currentChildMapping = this.props.children;
+			currentChildMapping = filterNullChildren(this.props.children);
 		if (showProp && currentChildMapping &&
 			isShownInChildrenByKey(currentChildMapping, key, showProp)) {
 			this.performEnter(key);
@@ -200,7 +200,7 @@ export class CSSTransitionGroup extends Component {
 	render({ component:Component, transitionName, transitionEnter, transitionLeave, children:c, ...props }, { children }) {
 		return (
 			<Component {...props}>
-				{ children.map(this.renderChild) }
+				{ filterNullChildren(children).map(this.renderChild) }
 			</Component>
 		);
 	}

src/util.js

@@ -9,3 +9,7 @@ export function getComponentBase(component) {
 export function onlyChild(children) {
 	return children && children[0];
 }
+
+export function filterNullChildren(children) {
+	return children && children.filter(i => i !== null);
+}