Adapt an empty return value from eval to match Script Editor expectations

Author: jpsacha

src/main/scala/org/scijava/plugins/scripting/scala/ScalaAdaptedScriptEngine.scala

@@ -25,7 +25,13 @@ class ScalaAdaptedScriptEngine(engine: ScriptEngine) extends AbstractScriptEngin
   @throws[ScriptException]
   override def eval(script: String, context: ScriptContext): AnyRef =
     emulateBinding(context)
-    evalInner(script, context)
+    val r = evalInner(script, context)
+    // Scala returns `Unit` when no value is returned. Script Engine (or the
+    // Java side) expects `null` when no value was returned.
+    // Anything else return as is.
+    r match
+      case _: Unit => null
+      case x       => x
 
   private def emulateBinding(context: ScriptContext): Unit =
 

src/test/java/org/scijava/plugins/scripting/scala/ScalaTest.java

@@ -74,6 +74,17 @@ public void testBasic() throws Exception {
         }
     }
 
+    @Test
+    public void testEmptyReturnValue() throws Exception {
+        try (final Context context = new Context(ScriptService.class)) {
+            final ScriptService scriptService = context.getService(ScriptService.class);
+            final ScriptModule m = scriptService.run("hello.scala", "print(\"3\")", true).get();
+            final Void expected = null;
+            final Object actual = m.getReturnValue();
+            assertEquals(expected, actual);
+        }
+    }
+
     @Test
     public void testPutDouble() throws Exception {
         try (final Context context = new Context(ScriptService.class)) {