fix(js): handle the component unmounting (#340)

* Fixes #339 - Handle the component unmounting

If the component unmounts immediately after mounting, then the call to
requestAnimationFrame will attempt to operate on the unmounted
component. We can prevent this by ensuring we cleanup the callback on
unmounting.

* Default value to undefined, rather than 0

Author: IPWright83

src/PagerView.tsx

@@ -55,12 +55,19 @@ import { getViewManagerConfig, PagerViewViewManager } from './PagerViewNative';
 
 export class PagerView extends React.Component<PagerViewProps> {
   private isScrolling = false;
+  private animationFrameRequestId?: number;
   private PagerView = React.createRef<typeof PagerViewViewManager>();
 
+  componentWillUnmount() {
+    if (this.animationFrameRequestId !== undefined) {
+      cancelAnimationFrame(this.animationFrameRequestId);
+    }
+  }
+
   componentDidMount() {
     // On iOS we do it directly on the native side
     if (Platform.OS === 'android' && this.props.initialPage !== undefined) {
-      requestAnimationFrame(() => {
+      this.animationFrameRequestId = requestAnimationFrame(() => {
         if (this.props.initialPage !== undefined) {
           this.setPageWithoutAnimation(this.props.initialPage);
         }