title: AdmobFree interstitial.show() - ERROR: interstitial not ready yet
body: <p>I first call</p>
  
  <pre><code>this.admobFree.interstitial.prepare()
      .then(() =&gt; {
        this.interstitialPrepared = true;
        console.log('AdMob Interstitial Ad is prepared, will be presented if autoShow is true, otherwise, call showInterstitial().');
      })
      .catch((err) =&gt; {
        console.error(err);
      })
  </code></pre>
  
  <p>And <code>this.interstitialPrepared = true;</code> is called so I assume my ad is ready.</p>
  
  <p>But if I call <code>this.admobFree.interstitial.show()</code> after my <code>this.interstitialPrepared</code> var switch to <code>true</code>, I still have the following error "ERROR: interstitial not ready yet.".</p>
  
  <p><strong>Notice :</strong> 
  It works well with the following config  : </p>
  
  <pre><code>this.adMobProvider.interstitialConfig = {
          autoShow: false,
          isTesting : true
  };
  </code></pre>
  
  <p>But not when I want to test with real ads</p>
  
  <pre><code>this.adMobProvider.interstitialConfig = {
          autoShow: false,
          isTesting : false,
          id:"ca-app-pub-277368299xxxxxxxx"
  };
  </code></pre>