correction to preemtReceiver in CoreObject

dream/simulation/CoreObject.py

@@ -271,7 +271,6 @@ class CoreObject(object):
     #===========================================================================
     def preemptReceiver(self):
         activeObjectQueue=self.Res.users
-    
         # find a critical order if any
         critical=False
         for entity in activeObjectQueue:
@@ -281,6 +280,8 @@ class CoreObject(object):
                 break
         if critical:
             # pick a receiver
+            receiver=None
+            if any(object for object in self.next if object.isPreemptive and object.checkIfActive()):
                 receiver=next(object for object in self.next if object.isPreemptive and object.checkIfActive())
             # if there is any receiver that can be preempted check if it is operated
             if receiver: