Fix: add checking for null to avoid exception when .setAdUnitId is set twice

Author: almostintuitive

android/src/main/java/com/sbugert/rnadmob/RNAdMobInterstitialAdModule.java

@@ -106,7 +106,9 @@ private void sendEvent(String eventName, @Nullable WritableMap params) {
 
  @ReactMethod
  public void setAdUnitID(String adUnitID) {
- mInterstitialAd.setAdUnitId(adUnitID);
+ if (mInterstitialAd.getAdUnitId() == null) {
+ mInterstitialAd.setAdUnitId(adUnitID);
+ }
  }
 
  @ReactMethod