fix: teardown happening during initial setup (#134)

## Description
This fixes an issue that can occur if the AdvertisingProvider unmounts
during the initial GPT setup.

I've added a new state value (isInitialSetupComplete), which gets set
when the setup completes and prevents `omponentWillUnmount` from calling
the teardown function. This ensures we don't create a race condition
between the setup and teardown, which can break the display ads.

Co-authored-by: Stephen Gill <stephen.gill@adevinta.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "react-advertising",
-  "version": "4.2.8",
+  "version": "4.2.9",
   "description": "Library for display ads in React applications",
   "main": "lib/index.js",
   "unpkg": "dist/react-advertising.min.js",

src/components/AdvertisingProvider.js

@@ -13,6 +13,7 @@ export default class AdvertisingProvider extends Component {
     this.state = {
       activate: this.advertising.activate.bind(this.advertising),
       config: this.props.config,
+      isInitialSetupComplete: false,
     };
   }
 
@@ -66,7 +67,11 @@ export default class AdvertisingProvider extends Component {
   }
 
   async componentWillUnmount() {
-    if (this.props.config) {
+    /**
+     * Prevent the teardown call while initial setup still in progress
+     * otherwise it can create a race condition that breaks the setup process
+     */
+    if (this.props.config && this.state.isInitialSetupComplete) {
       await this.teardown();
     }
   }