title: Gradle: Could not get unknown property 'classesDir' for main classes
body: <p>According to the <a href="https://plugins.gradle.org/plugin/aspectj.gradle" rel="noreferrer">documentation</a>, I've tried to use aspectj plugin.</p>
  
  <p>This is the message I get when I build my project.</p>
  
  <pre><code>FAILURE: Build failed with an exception.
  
  * Where:
  Build file '/home/jesudi/projects/gradle-vscode/build.gradle' line: 22
  
  * What went wrong:
  A problem occurred evaluating root project 'security'.
  &gt; Failed to apply plugin [id 'aspectj.gradle']
     &gt; Could not create task ':compileAspect'.
        &gt; Could not get unknown property 'classesDir' for main classes of type org.gradle.api.internal.tasks.DefaultSourceSetOutput.
  </code></pre>
  
  <p>This is my script:</p>
  
  <pre><code>buildscript {
      repositories {
          mavenCentral()
          maven {
              url "https://plugins.gradle.org/m2/"
          }
      }
      dependencies {
          classpath "org.apache.meecrowave:meecrowave-gradle-plugin:1.2.6"
          classpath "gradle.plugin.aspectj:gradle-aspectj:0.1.6"
      }
  }
  
  plugins {
      id 'java'
  }
  
  project.ext {
      aspectjVersion = '1.9.2'
  }
  
  apply plugin: 'aspectj.gradle'
  apply plugin: "org.apache.microwave.microwave"
  
  java {
      sourceCompatibility = JavaVersion.VERSION_11
      targetCompatibility = JavaVersion.VERSION_11
  }
  
  dependencies {
      compile("org.apache.meecrowave:meecrowave-core:1.2.6")
      compile("org.apache.meecrowave:meecrowave-specs-api:1.2.6")
  }
  
  meecrowave {
      httpPort = 9090
      // most of the meecrowave core configuration
  }
  </code></pre>
  
  <p>This is the <code>gradle -version</code> output:</p>
  
  <pre><code>------------------------------------------------------------
  Gradle 5.1.1
  ------------------------------------------------------------
  
  Build time:   2019-01-10 23:05:02 UTC
  Revision:     3c9abb645fb83932c44e8610642393ad62116807
  
  Kotlin DSL:   1.1.1
  Kotlin:       1.3.11
  Groovy:       2.5.4
  Ant:          Apache Ant(TM) version 1.9.13 compiled on July 10 2018
  JVM:          11.0.2 (Oracle Corporation 11.0.2+9)
  OS:           Linux 4.15.0-20-generic amd64
  </code></pre>