I am using lasted version of Android Studio, and i have obfuscated my project by ProGuard in Android Studio. 
edit content of Build.gradlesuch as this : 
buildTypes {
    release {
        minifyEnabled true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
but when start the build project, an error like that 

warning Image : 
How can I fix this?
                        
I had same issue,try to change minifyEnabled true to
minifyEnabled false.And just refer this enter link description here