MissingPropertyException for access to private static field from inner class when subclassing

class Main extends Outer {
  static main(args) {
    newInstance().newThread()
  }
}

abstract class Outer {
  private static volatile boolean flag

  protected void newThread() {
    Thread thread = new Inner()
    thread.start()
    thread.join()
  }

  private final class Inner extends Thread {
    void run() {
     try {
      if (!flag) {
        sleep(1000)
      }
     } catch (Throwable t) {
      t.printStackTrace()
     }
    }
  }
}

groovy.lang.MissingPropertyException: No such property: flag for class: Main
Possible solutions: class
	at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:65)
	at org.codehaus.groovy.runtime.callsite.GetEffectivePogoPropertySite.getProperty(GetEffectivePogoPropertySite.java:87)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callGroovyObjectGetProperty(AbstractCallSite.java:309)
	at Outer$Inner.run(Script1.groovy:20)