title: Android Studio : Could not get unknown property 'kotlin_version'
body: <p>After git cloning from <a href="https://github.com/VoiceGym/voicegym" rel="nofollow noreferrer">here</a>,</p>
  <p>I first faced with this error:</p>
  <pre><code>1: Task failed with an exception.
  -----------
  * Where:
  Build file 'C:\...\voicegym\app\build.gradle' line: 3
  
  * What went wrong:
  A problem occurred evaluating project ':app'.
  &gt; Could not initialize class org.jetbrains.kotlin.gradle.internal.KotlinSourceSetProviderImplKt
  </code></pre>
  <p>following <a href="https://stackoverflow.com/questions/60833542/kotlin-gradle-could-not-initialize-class-class-org-jetbrains-kotlin-gradle-inter">this advice</a>, i added these to <code>build.gradle (Project: voicegym)</code></p>
  <pre class="lang-java prettyprint-override"><code>buildscript {
      ext.kotlinVersion = '1.3.72' // previously  '1.2.60'
      ext.springBootVersion = '2.0.4.RELEASE'
      ext.anko_version='0.10.4'
      repositories {
          ...
          mavenCentral()
      }
      dependencies {
          classpath 'com.android.tools.build:gradle:4.1.1'
          classpath &quot;org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version&quot;
      }
  }
  </code></pre>
  <p>Now, i have this error</p>
  <pre><code>Build file 'C:\...\voicegym\build.gradle' line: 5
  
  A problem occurred evaluating root project 'voicegym'.
  &gt; Could not get unknown property 'kotlin_version' for object of type org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.
  </code></pre>