[admob][ios] Fix an issue whereby interstitials on iOS could not be reused with the same adUnit ID

lib/modules/admob/Interstitial.js

@@ -1,4 +1,4 @@
-import { NativeModules } from 'react-native';
+import { NativeModules, Platform } from 'react-native';
 import { statics } from './';
 import AdRequest from './AdRequest';
 import { nativeToJSError } from '../../utils';
@@ -8,6 +8,11 @@ const FirebaseAdMob = NativeModules.RNFirebaseAdMob;
 export default class Interstitial {
 
   constructor(admob: Object, adUnit: string) {
+    // Interstitials on iOS require a new instance each time
+    if (Platform.OS === 'ios') {
+      FirebaseAdMob.clearInterstitial(adUnit);
+    }
+
     this.admob = admob;
     this.adUnit = adUnit;
     this.loaded = false;