Bugfix NPE in ClientAssociation#close()

Added null check for reportListener

src/main/java/com/beanit/iec61850bean/ClientAssociation.java

@@ -2110,15 +2110,17 @@ public final class ClientAssociation {
           closed = true;
           acseAssociation.close();
           lastIOException = e;
-          Thread t1 =
-              new Thread(
-                  new Runnable() {
-                    @Override
-                    public void run() {
-                      reportListener.associationClosed(lastIOException);
-                    }
-                  });
-          t1.start();
+          if (reportListener != null) {
+            Thread t1 =
+                new Thread(
+                    new Runnable() {
+                      @Override
+                      public void run() {
+                        reportListener.associationClosed(lastIOException);
+                      }
+                    });
+            t1.start();
+          }
 
           MMSpdu mmsPdu = new MMSpdu();
           mmsPdu.setConfirmedRequestPDU(new ConfirmedRequestPDU());