Don't initialize song visualizer when not needed

Author: RaphiMC

src/main/java/net/raphimc/noteblocktool/frames/SongPlayerFrame.java

@@ -303,7 +303,9 @@ public void windowClosing(WindowEvent e) {
             public void windowClosed(WindowEvent e) {
                 SongPlayerFrame.this.songPlayer.stop();
                 SongPlayerFrame.this.updateTimer.stop();
-                VisualizerWindow.getInstance().hide();
+                if (VisualizerWindow.hasInstance()) {
+                    VisualizerWindow.getInstance().hide();
+                }
                 if (SongPlayerFrame.this.soundSystem != null) SongPlayerFrame.this.soundSystem.close();
             }
         });

src/main/java/net/raphimc/noteblocktool/frames/visualizer/VisualizerWindow.java

@@ -47,6 +47,10 @@ public static VisualizerWindow getInstance() {
         return INSTANCE;
     }
 
+    public static boolean hasInstance() {
+        return INSTANCE != null;
+    }
+
     private VisualizerWindow() {
         this.renderThread = new Thread(() -> {
             GLFWErrorCallback.createPrint(System.err).set();