title: The mapper function returned a null value
body: <p>I set same build types for debug and release,</p>
  
  <pre><code>buildTypes {
      debug {
          buildConfigField "String", "API_BASE_URL", "\"https://www.testUrl.com/api/\""
          minifyEnabled true
          shrinkResources true
          proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          signingConfig signingConfigs.release_key
      }
      release {
          buildConfigField "String", "API_BASE_URL", "\"https://www.testUrl.com/api/\""
          minifyEnabled true
          shrinkResources true
          proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          signingConfig signingConfigs.release_key
      }
  }
  </code></pre>
  
  <p>But If I build with the release, I got the below error. 
  Also, the server response is exactly same.</p>
  
  <pre><code>W/System.err: java.lang.NullPointerException: The mapper function returned a null value.
  W/System.err:     at b.a.e.b.b.a(Unknown Source)
  W/System.err:     at b.a.e.e.b.bs$a.onNext(Unknown Source)
  W/System.err:     at b.a.e.e.b.cm$a.onNext(Unknown Source)
  W/System.err:     at retrofit2.adapter.rxjava2.BodyObservable$BodyObserver.onNext(Unknown Source)
  W/System.err:     at retrofit2.adapter.rxjava2.BodyObservable$BodyObserver.onNext(Unknown Source)
  W/System.err:     at retrofit2.adapter.rxjava2.CallExecuteObservable.subscribeActual(Unknown Source)
  W/System.err:     at b.a.l.subscribe(Unknown Source)
  W/System.err:     at retrofit2.adapter.rxjava2.BodyObservable.subscribeActual(Unknown Source)
  W/System.err:     at b.a.l.subscribe(Unknown Source)
  W/System.err:     at b.a.e.e.b.cm$a.a(Unknown Source)
  W/System.err:     at b.a.e.e.b.cm.subscribeActual(Unknown Source)
  W/System.err:     at b.a.l.subscribe(Unknown Source)
  W/System.err:     at b.a.e.e.b.bs.subscribeActual(Unknown Source)
  W/System.err:     at b.a.l.subscribe(Unknown Source)
  W/System.err:     at b.a.e.e.b.bw.subscribeActual(Unknown Source)
  W/System.err:     at b.a.l.subscribe(Unknown Source)
  W/System.err:     at b.a.e.e.b.dd$b.run(Unknown Source)
  W/System.err:     at b.a.s$a.run(Unknown Source)
  W/System.err:     at b.a.e.g.j.run(Unknown Source)
  W/System.err:     at b.a.e.g.j.call(Unknown Source)
  W/System.err:     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
  W/System.err:     at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:272)
  W/System.err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
  W/System.err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
  W/System.err:     at java.lang.Thread.run(Thread.java:761)
  </code></pre>