Bugfix NPE in ClientAssociation#close()

Added null check for reportListener

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

@@ -2110,6 +2110,7 @@ public final class ClientAssociation {
           closed = true;
           acseAssociation.close();
           lastIOException = e;
+          if (reportListener != null) {
             Thread t1 =
                 new Thread(
                     new Runnable() {
@@ -2119,6 +2120,7 @@ public final class ClientAssociation {
                       }
                     });
             t1.start();
+          }
 
           MMSpdu mmsPdu = new MMSpdu();
           mmsPdu.setConfirmedRequestPDU(new ConfirmedRequestPDU());