Ensure we create a plot window object to the test

Author: aherbert

gdsc-core-ij/src/test/java/uk/ac/sussex/gdsc/core/ij/plugin/WindowOrganiserTest.java

@@ -86,7 +86,8 @@ void testAdd() {
   @DisabledIfHeadless
   void testAdd2() {
     final WindowOrganiser wo = new WindowOrganiser();
-    final PlotWindow pw = new Plot("dummy", "x", "y").show();
+    final Plot plot = new Plot("dummy", "x", "y");
+    final PlotWindow pw = new PlotWindow(plot.getImagePlus(), plot);
     wo.setIgnore(true);
     wo.add(pw);
     Assertions.assertTrue(wo.isEmpty());