Fixes #1249 - Don't hang up when destroy is called multiple times (#1250)

jme3-lwjgl/src/main/java/com/jme3/system/lwjgl/LwjglAbstractDisplay.java

@@ -267,6 +267,10 @@ public abstract class LwjglAbstractDisplay extends LwjglContext implements Runna
     }
 
     public void destroy(boolean waitFor) {
+        if (needClose.get()) {
+            return; // Already destroyed
+        }
+
         needClose.set(true);
         if (waitFor)
             waitFor(false);