title: Trouble call fails with Attempt to invoke interface method 'java.lang.reflect.Type retrofit2.CallAdapter.responseType()' on a null object reference
body: <p>I'm having trouble doing a retrofit request that returns a simple Call object.</p>
  
  <p>I have my retrofit setup done this way:</p>
  
  <pre><code> new Retrofit.Builder()
                  .addCallAdapterFactory(RxErrorHandlingCallAdapterFactory.create())
                  .addConverterFactory(GsonConverterFactory.create(gson));
  </code></pre>
  
  <p>And I have the following retrofit interface which has mixed rxjava and simple calls defined</p>
  
  <pre><code>MyInterface {
     Single getSingle1();
     Single getSingle2();
     Call getCall1();
  }
  </code></pre>
  
  <p>The issue that I face is when I execute:
  <code>myInterface.getCall1().execute()</code> I get the following error</p>
  
  <pre><code>Attempt to invoke interface method 'java.lang.reflect.Type retrofit2.CallAdapter.responseType()' on a null object reference
  </code></pre>
  
  <p>Should be noted that if I convert <code>Call getCall1()</code> to return <code>Completable getCall1()</code> everything works as expected.</p>
  
  <p>Anyone has any idea of what I'm doing wrong ?</p>