fix(admob, ios): null check interstitialAd on show (#4670)

Author: Guglio

packages/admob/android/src/main/java/io/invertase/firebase/admob/ReactNativeFirebaseAdMobInterstitialModule.java

@@ -125,6 +125,10 @@ public void interstitialShow(int requestId, ReadableMap showOptions, Promise pro
     }
     getCurrentActivity().runOnUiThread(() -> {
       InterstitialAd interstitialAd = interstitialAdArray.get(requestId);
+      if (interstitialAd == null) {
+        rejectPromiseWithCodeAndMessage(promise, "null-interstitialAd", "Interstitial ad attempted to show but its object was null.");
+        return;
+      }
 
       if (showOptions.hasKey("immersiveModeEnabled")) {
         interstitialAd.setImmersiveMode(showOptions.getBoolean("immersiveModeEnabled"));