title: PITest Issue: property 'mainClass' is final and cannot be changed any further
body: <p>I have recently upgraded my project from <code>Java 8</code> to <code>Java 11</code> alongside <code>Gradle 6.5</code>.
  However, when I run the command <code>./gradlew pitest</code> I receive the following error:</p>
  <pre><code>Execution failed for task ':pitest'.
  &gt; The value for task ':pitest' property 'mainClass' is final and cannot be changed any further.
  </code></pre>
  <p>Here is my <code>pitest</code> set-up in my <code>build.gradle</code>:</p>
  <pre><code>pitest {
      pitestVersion = '1.4.3'
      targetClasses = ['com.myproject.*']
      excludedClasses = [
              'com.myproject.configuration.*',
              'com.myproject.controller.*',
      ]
      threads = 10
      enableDefaultIncrementalAnalysis = true
      historyInputLocation = ['build/reports/pitest/fastermutationtesting']
      historyOutputLocation = ['build/reports/pitest/fastermutationtestingoutput']
      outputFormats = ['XML', 'HTML']
      timestampedReports = true
      mutationThreshold = 90   
  }
  </code></pre>
  <p>and here are my <code>pitest</code> related dependencies:</p>
  <pre><code>Plugins{
         id &quot;info.solidsoft.pitest&quot; version '1.3.0'
  }
  
  testCompile 'info.solidsoft.gradle.pitest:gradle-pitest-plugin:1.3.0'
  </code></pre>
  <p>I am not sure where this <code>mainClass</code> property is coming from and have not found any related answers or documentation on this issue.</p>