Do not extend App to avoid NPE in Example

`Example extends App` was causing a `NullPointerException` when running
`Example.image.draw` from the console.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/main/scala/Example.scala b/src/main/scala/Example.scala
index ac72a34..e485b9a 100644
--- a/src/main/scala/Example.scala
+++ b/src/main/scala/Example.scala
@@ -7,7 +7,10 @@ import doodle.backend.StandardInterpreter._
 // To use this example, open the SBT console and type:
 //
 // Example.image.draw
-object Example extends App {
+object Example {
   val image = circle(10).fillColor(Color.red) on circle(20) on circle(30)
-  image.draw
+
+  def main(args: Array[String]): Unit = {
+    image.draw
+  }
 }