Android 4.X : NoClassDefFoundError: io/reactivex/Observable

Question

I am using AWS Dynamodb database with Android application. Application is working perfect with higher Android version.

SDK version of dynamodb is com.amazonaws:aws-android-sdk-ddb-mapper:2.6.15

DynamoDBMapper.save(object); // Crash on this line

Facing issue only with kitkat and below version of android.

My application gradle file.

    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;
    }
}

This question is not duplicate of Android Gradle Multidex Build issue on API 19. 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.