fix bug with queues dictionary access

shared/workspaces/workspaces/capability/services/capability_queue.py

@@ -99,9 +99,10 @@ class CapabilityQueue(CapabilityQueueIF):
         :param execution_id: ID of requested execution
         """
         engine = self.find_engine(execution_id)
-        self.engine_list.in_use.remove(engine)
-        engine.execution = None
-        self.engine_list.available.put(engine)
+        if engine:
+            self.engine_list.in_use.remove(engine)
+            engine.execution = None
+            self.engine_list.available.put(engine)
 
     def get_available_engine(self) -> Optional[CapabilityEngine]:
         """