Fix possible exception with ReactDOM.unmountComponentAtNode (#24)

Author: herzog31

lib/WebComponent.js

@@ -110,7 +110,11 @@ export class CustomElement extends HTMLElement {
     }
 
     disconnectedCallback() {
-		ReactDOM.unmountComponentAtNode(_rootShadows.get(this));
+        const rootEl = _rootShadows.get(this);
+        if(rootEl) {
+            ReactDOM.unmountComponentAtNode(_rootShadows.get(this));
+        }
+
 		if (this.rootDiv) {
 			this.removeChild(this.rootDiv);
 			delete this.rootDiv;