Check error stream is not null before notifying error (#961)

Co-authored-by: Jeroen van Erp <jeroen@hierynomus.com>

src/main/java/net/schmizz/sshj/connection/channel/direct/SessionChannel.java

@@ -225,7 +225,9 @@ public class SessionChannel
 
     @Override
     public void notifyError(SSHException error) {
-        err.notifyError(error);
+        if (err != null) {
+            err.notifyError(error);
+        }
         super.notifyError(error);
     }