Open statistics dialog from correct thread

The dialog to ask for collecting anonymous statistics needs to be created in
the Swing EDT. Do so using SwingUtilities.invokeLater().

This is a follow-up on 7ef2b3a and should finally fix JabRef#2955.

src/main/java/org/jabref/gui/JabRefFrame.java

@@ -664,7 +664,12 @@ private void initShowTrackingNotification() {
 
                 @Override
                 public void run() {
-                    DefaultTaskExecutor.runInJavaFXThread(JabRefFrame.this::showTrackingNotification);
+                    SwingUtilities.invokeLater(new Runnable() {
+                        @Override
+                        public void run() {
+                            DefaultTaskExecutor.runInJavaFXThread(JabRefFrame.this::showTrackingNotification);
+                        }
+                    });
                 }
             }, 60000); // run in one minute
         }