title: Android 4.X : NoClassDefFoundError: io/reactivex/Observable
body: <p>I am using AWS Dynamodb database with Android application. Application is working perfect with higher Android version.</p>
  
  <p>SDK version of dynamodb is <strong>com.amazonaws:aws-android-sdk-ddb-mapper:2.6.15</strong></p>
  
  <pre><code>DynamoDBMapper.save(object); // Crash on this line
  </code></pre>
  
  <p>Facing issue only with kitkat and below version of android.</p>
  
  <p>My application gradle file.</p>
  
  <pre><code>    buildscript {
      repositories {
          maven { url 'https://maven.fabric.io/public' }
      }
  
      dependencies {
          classpath 'io.fabric.tools:gradle:1.+'
      }
  }
  apply plugin: 'com.android.application'
  apply plugin: 'io.fabric'
  
  repositories {
      maven { url 'https://maven.fabric.io/public' }
  }
  
  apply plugin: 'realm-android'
  android {
  
      compileSdkVersion 27
      defaultConfig {
          minSdkVersion 16
          targetSdkVersion 27
          vectorDrawables.useSupportLibrary = true
          multiDexEnabled true
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
              debuggable true
          }
      }
      compileOptions {
          sourceCompatibility JavaVersion.VERSION_1_8
          targetCompatibility JavaVersion.VERSION_1_8
      }
  }
  
  dependencies {
      implementation 'com.google.firebase:firebase-messaging:11.8.0'
      implementation 'com.google.firebase:firebase-crash:11.8.0'
      implementation 'com.google.android.gms:play-services-maps:11.8.0'
      implementation 'com.google.android.gms:play-services-places:11.8.0'
      implementation 'io.reactivex:rxjava:1.3.6'
      implementation 'jp.wasabeef:glide-transformations:3.1.0'
      implementation 'com.facebook.android:facebook-login:4.31.0'
      implementation 'com.android.support:multidex:1.0.3'
      implementation "com.android.support:cardview-v7:$support_version"
      implementation "com.android.support:appcompat-v7:$support_version"
      implementation "com.android.support:design:$support_version"
      implementation "com.android.support:recyclerview-v7:$support_version"
      implementation 'com.android.support.constraint:constraint-layout:1.0.2'
      implementation 'com.google.android.gms:play-services-auth:11.8.0'
      implementation 'com.dlazaro66.qrcodereaderview:qrcodereaderview:2.0.3'
      implementation 'com.amazonaws:aws-android-sdk-ddb-mapper:2.6.15'
      implementation 'com.amazonaws:aws-android-sdk-s3:2.6.15'
      implementation 'com.amazonaws:aws-android-sdk-lambda:2.6.15'
      implementation 'com.google.zxing:core:3.3.0'
      implementation 'org.elasticsearch.client:elasticsearch-rest-client-sniffer:6.2.1'
      implementation 'com.devbrackets.android:exomedia:4.1.0'
      implementation 'com.github.chrisbanes:PhotoView:2.1.3'
      compile('com.crashlytics.sdk.android:crashlytics:2.9.0@aar') {
          transitive = true;
      }
  }
  </code></pre>
  
  <p>This question is not duplicate of <a href="https://stackoverflow.com/questions/32816964/android-gradle-multidex-build-issue-on-api-19">Android Gradle Multidex Build issue on API 19</a>. Because my app is working fine in above version of Kitkat and also I have not faced any Gradle build issue. Application is running but it got crash while I am saving model object into dynamodb with Dynamodb mapper class.</p>