SerialMonitor: dispose its window before setting its var to null, otherwise dangling disabled windows may occur. Fixes #3379

Author: Federico Fissore

app/src/processing/app/Editor.java

@@ -137,7 +137,7 @@ public boolean apply(Sketch sketch) {
 
   private static JMenu portMenu;
 
-  static AbstractMonitor serialMonitor;
+  static volatile AbstractMonitor serialMonitor;
 
   final EditorHeader header;
   EditorStatus status;
@@ -2487,6 +2487,7 @@ public void handleSerial() {
       // The serial monitor already exists
 
       if (serialMonitor.isClosed()) {
+        serialMonitor.dispose();
         // If it's closed, clear the refrence to the existing
         // monitor and create a new one
         serialMonitor = null;