Check if application running only if 'Class' key specified in config.

diff --git a/qwerty.py b/qwerty.py
index c0bb591..775ba59 100755
--- a/qwerty.py
+++ b/qwerty.py
@@ -273,8 +273,9 @@ class GUI(object):
             btn = self.buttons[k]
             btn.set_label(K.title)
             btn.set_icon(K.icon)
-            if K.cls in self.by_class:
-                btn.mark_running(K.cls)
+            if hasattr(K,'cls'):
+              if K.cls in self.by_class:
+                  btn.mark_running(K.cls)

 parse_config()